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PRESENTED  BY 

PROF. CHARLES  A.  KOFOID  AND 
MRS.  PRUDENCE  W.  KOFOID 


KARTO 
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CONVERGENCE    IN    EVOLUTION 


FIG.  7  (see  p.  90).     HEAD  OF  INDIAN  MUNGOOSE  (Hcrpcstes 
smithii\  showing  horizontal  pupil. 


CONVERGENCE  IN 
EVOLUTION 


BY  ARTHUR  WILLEY 

D.Sc.,  LOND.;  HON.  M.A.,  CANTAB.;  F.R.S. 

SOMETIME  BALFOUR   STUDENT  OF  THE   UNIVERSITY   OF   CAMBRIDGE  ;    LATE 

DIRECTOR  OF  THE   COLOMBO   MUSEUM,    CEYLON;    STRATHCONA 

PROFESSOR  OF  ZOOLOGY  IN  THE  M'GILL  UNIVERSITY, 

MONTREAL,  CANADA;   AUTHOR  OF  "  AMPHIOXUS 

AND  THE   ANCESTRY  OF  VERTEBRATES  " 


WITH   ILLUSTRATIONS 


LONDON 

JOHN  MURRAY,  ALBEMARLE  STREET,  W. 

1911 


Gift  of  C.  A.  Kofoid 


EARTH 

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LIBRARY 


DEDICATION 

To  SIR  E.  RAY  LANKESTER,  K.C.B.,  LL.D.,  F.R.S. 

DEAR  SIR  RAY, 

Twenty  years  have  elapsed  since  I 
published  my  first  preliminary  note  under  your 
guidance,  and  sixteen  since  I  endeavoured  to  make 
a  scanty  return  by  dedicating  a  volume  to  you. 

If  there  was  a  reason  then,  there  must  be  a  two- 
fold reason  now  for  associating  your  name  with  a 
book  of  this  nature,  the  one  to  balance  the  other. 

To  you,  with  your  intimate  and  apostolic  know- 
ledge of  the  situation,  and  mastery  of  the  facts  which 
govern  it,  it  may  seem  rash  to  attempt  anything 
like  a  revision  of  first  principles ;  but  having  regard 
to  the  actual  state  of  zoological  speculation,  yesterday 
or  to-day,  it  is  clear  that  much  remains  to  be  done 
before  the  ground  can  be  freed  from  many  of  the 
qualities  which  favour  the  growth  of  error. 

Yours  sincerely, 

A.  WILLEY. 


a  2 


Kj  jdb  r~ 


EARTH 

SCIENCES 

LFBRARY 

PREFACE 

IN  these  pages  I  have  attempted  to  place  on 
record  in  a  connected  form  some  facts  of  natural 
history  which  have,  directly  or  indirectly,  passed 
under  my  own  observation  during  a  number 
of  years,  together  with  others  which  I  have 
collected  from  various  sources.  Many  of  them 
are  known  facts  brought  into  fresh  conjunctions, 
others  are  somewhat  less  familiar,  and  some  are 
practically  new.  I  have  endeavoured  to  render 
the  language  intelligible  to  those  who  have 
an  inkling  of  biological  knowledge;  and  terms 
which  may  appear  difficult  in  the  text  are 
usually  explained  in  the  context;  but  I  have 
found  it  impossible  to  avoid  using  the  current 
phraseology  of  the  subject. 

Morphology  as  comprehending  the  analysis  of 
organic  form  is  distinguished  from  art  which 
concerns  itself  with  the  portrayal  of  form ;  and 
it  appeals  to  a  much  more  limited  circle,  its 
applications  being  obscure  and,  as  a  general 
rule,  of  interest  only  to  specialists.  Its  relation- 


vn 


M335738 


viii  PREFACE 

ship  to  art  is  obvious  in  many  of  its  aspects, 
not  least  so  in  its  bearing  towards  truth.  The 
flamboyant  manner  in  which  Napoleon  crosses 
the  Alps  on  canvas  is  not  farther  from  the 
truth  than  many  a  morphological  theory  which 
has  received  the  last  touches  of  a  master  hand. 
Yet  the  theory  and  the  picture  though  wrong 
are  presumably  good,  that  is  to  say,  the  technique 
is  competent,  and  the  moment  selected  for  inter- 
pretation and  presentation  interesting ;  and  if 
there  is  a  preference  it  must  be  for  the  picture 
which  makes  no  pretension  to  scientific  accuracy. 

Beyond  a  certain  point  morphology  becomes 
an  art,  and  as  such  its  scope  is  infinite ;  as  an 
expression  of  simple  truth  its  horizon  is  strictly 
limited,  and  consequently  there  have  not  been 
wanting  those  who  have  depreciated  its  value, 
perhaps  through  forgetfulness  of  the  fact  that 
the  complexity  of  biological  questions  is  rarely 
realised  even  by  specialists,  because  the  origins 
of  morphological  types  are  rooted  in  remote 
and  intangible  obscurity.  It  should  be  added 
that  my  remarks  refer  almost  exclusively  to 
the  zoological  side  of  morphology,  although 
many  of  the  principles  involved  apply  with 
equal,  if  not  with  greater  force  to  its  botanical 
aspect. 

The   history   of  zoological  classification  since 


PREFACE  ix 

the  time  of  Linnaeus  has  consisted  largely  of 
rectifications  in  the  systematic  positions  origin- 
ally assigned  to  animals  which  were  thought 
to  resemble  others  in  certain  superficial  traits 
of  their  organisation,  and  hence  were  classed 
together,  e.g.,  the  Foraminifera  and  the  Nauti- 
loidea,  the  Cephalopoda  and  Pteropoda,  the 
Cirripedia,  Tunicata  and  Mollusca.  It  is  only 
after  such  groups  are  relegated  to  their  proper 
position  in  the  zoological  system  that  the 
phenomena  of  convergence  begin  to  appear  in 
their  true  light  with  an  independent  interest 
of  their  own.  The  limitations  of  convergence 
coincide  with  those  of  homology,  and  the  criteria 
of  the  one  are  inversely  those  of  the  other. 
Its  importance  in  morphology  is  therefore  clear 
enough,  and  no  apology  would  be  required  for 
the  repeated  discussion  of  it,  were  the  treatment 
adequate. 

Up  to  a  certain  point  the  gist  of  what  I  have 
set  down  in  these  pages  may  be  regarded  as 
an  attempt  at  a  reply  to  a  recent  "  earthquake 
hypothesis  "  concerning  the  origin  of  Vertebrates, 
which  has  been  published  in  a  remarkable  volume 
by  a  very  eminent  physiologist,  Dr  W.  H. 
Gaskell.  I  can  only  hope  that  Dr  Gaskell  and 
others  will  accept  it  in  the  spirit  in  which  it  is 
offered.  Besides  this  I  have  endeavoured  to 


x  PREFACE 

suggest  in  outline  a  system  of  convergence 
which  is  capable  of  further  expansion ;  and  I 
have  made  a  point,  so  far  as  my  purview  ex- 
tended, of  acknowledging  my  indebtedness  to 
the  work  of  predecessors  and  contemporaries. 

The  technical  expressions  which  are  scattered 
through  the  text  will  be  found  not  to  interfere 
with  the  comprehension  of  the  whole.  They 
constitute  part  of  the  scheme  of  arrangement 
proposed  for  the  phenomena  of  convergence. 
Phenomena  require  to  be  classified  after  the 
same  manner  as  animals,  rocks  and  plants ; 
and  the  pregnant  technical  terms  are  intended 
to  carry  this  out  for  future  use  and  reference, 
not  for  the  bewilderment  of  the  reader. 

This  opuscule  was  planned  and  the  bulk  of 
the  manuscript  written  in  Ceylon.  After  my 
return  to  England  in  the  summer  of  this  year, 
and  after  the  manuscript  had  been  sent  to  the 
printer,  my  attention  was  drawn  by  a  friend 
to  an  important  paper  by  Professor  Henry 
Fairfield  Osborn  on  "  Homoplasy  as  a  Law  of 
Latent  or  Potential  Homology  "  in  the  American 
Naturalist,  vol.  xxxvi.,  1902,  pp.  259-271.  This 
paper  is  illustrated  by  examples  of  convergence 
in  tooth-structure  (dental  convergence),  and  con- 
tains a  valuable  exposition  of  homoplasy  from  the 
palaeontological  standpoint.  The  only  point  in 


PREFACE  xi 

which  I  find  myself,  unfortunately,  somewhat  at 
variance  with  Professor  Osborn  is  in  the  matter 
of  terminology. 

I  have  used  the  word  convergence  in  a  wide 
sense,  embracing  habits,  functions,  structure,  and 
physiognomy ;  and  I  venture  to  hope  that,  after 
due  consideration,  this  extended  application  will 
prove  acceptable.  Under  the  heading  "Analogy 
in  Evolution,"  Professor  Osborn  tabulates  four 
categories,  namely,  Analogous  Variation,  Paral- 
lelism, Convergence,  and  Homoplasy,  each  of 
which  he  defines.  Convergence  is  there  defined 
as  "  independent  similar  development  of  un- 
related animals,  bringing  them  apparently  closer 
together " ;  and  Parallelism  is  "  independent 
similar  development  of  related  animals,  plants, 
and  organs." 

These  definitions  leave  us  in  the  dark  as  to 
what  degrees  of  relationship  would  entitle  a 
given  case  to  be  classed  as  one  of  parallelism 
or  of  convergence.  The  four  classes  represent 
degrees  of  one  general  phenomenon,  and  the 
term  Analogy  seems  to  be  rather  too  vague 
as  applied  to  it.  For  the  rest  I  refer  the 
reader  to  what  I  have  said  on  the  subject  of 
degrees  of  convergence  and  to  the  diagrams 
of  convergence  and  parallelism. 

Professor  Gilbert  C.  Bourne  (in  the  Quarterly 


xii  PREFACE 

Journal  of  Microscopical  Science,  vol.  lv.,  1910, 
see  pp.  36-43),  also  finds  difficulty  in  differenti- 
ating between  Osborn's  categories,  and  adds 
that  "  Lankester's  term,  homoplasy,  as  origin- 
ally defined  [1870],  covers  all  the  cases." 

In  the  following  pages  I  have  tried  to  show 
that  homoplasy  does  not  cover  all  the  cases 
which  are  included  under  convergence  in  the 
wider  acceptation  of  the  term. 

A.   WILLEY. 


LONDON,  15^  August  1910. 


CONTENTS 


CHAP.  PAGE 

PREFACE vii 

I.  THE   ART   OF    MORPHOLOGY,   BEING   A   DISCOURSE 

UPON  ORGANIC  FORM I 

II.  PHYSIOLOGICAL  CLASSIFICATION 13 

III.  EXPOSED  AND   CONCEALED  ANIMALS  (PHANEROZOA 

AND  CRYPTOZOA) 23 

IV.  FREE  AND    FIXED   ANIMALS   (ELEUTHEROZOA   AND 

STATOZOA) 35 

V.  MIMICRY  AND  HOMOPLASY $2 

VI.  DIVERGENCE  AND  PARALLELISM 69 

VII.  SPECIAL  CONVERGENCE 87 

VIII.  HABITUDES    AND    ATTITUDES    (BIONOMICAL    CON- 
VERGENCE)        113 

IX.  THE    WAYS    OF    BREATHING    (RESPIRATORY    CON- 
VERGENCE)        138 

X.  CONVERGENCE    IN    MINUTE     STRUCTURE    (HISTO- 

GENETIC  CONVERGENCE) 153 

INDEX 173 


111 


LIST   OF   ILLUSTRATIONS 

HEAD      OF      INDIAN      MUNGOOSE      (HERPESTES     SMITHII\ 

SHOWING  HORIZONTAL  PUPIL     .        .        .       Frontispiece 

PAGE 
LEAF-FISH 66 

DIAGRAM  OF  PHYLETIC  DIVERGENCE  AND  PARALLELISM        .        70 
PARALLEL  DIVISIONS  OF  ECHINODERMATA    .  .          .          .71 

PARALLEL  DIVISIONS  OF  VERTEBRATA 72 

PARALLELISM  WITH  CONVERGENCE  AMONGST  MAMMALS     .        80 
HEAD  OF  DRYOPHIS  PULVERULENTUS  IN  SIDE  VIEW  .        93 

EYE    OF    DUSSUMIERIA,    SHOWING    PERFORATED    ADIPOSE 

EYELID 95 

INTESTINAL  TRACT  OF  DUSSUMIERIA  .          .          .          .      log 

DISSECTION  OF   THE  ALIMENTARY   TRACT   OF   CHATOESSUS 

NASUS  FROM  THE  LEFT  SIDE IIO 

PARALLELISM  WITH  CONVERGENCE  BETWEEN  TWO  FAMILIES 

OF   FISHES,   MUGILIDjE  AND  CLUPEID^        .          .  .112 

HEAD  AND   FORE-BODY  OF  ICHTHYOPHIS  GLVTINOSUS,   TO 

SHOW  THE  COLLAR-LIKE  RESPIRATORY   REGION         .      143 


CONVERGENCE    IN    EVOLUTION 
CHAPTER  I 

THE   ART   OF    MORPHOLOGY,    BEING   A   DISCOURSE 
UPON    ORGANIC    FORM 

MORPHOLOGY,  in  the  modern  sense,  usually  con- 
veys a  genetic  meaning,   implying  morphogeny 
or  the  origin  of  forms.     Its  purpose  is  to  follow 
the  clue  of  affinity  which   connects  related   but 
divergent   classes    of    animals    and    systems   of 
organs,   and   to   recognise   primary   forms  when 
disguised   beneath   secondary   facies.     It   is   the 
handmaiden   of  comparative   anatomy   which   is 
a  much  older  discipline,  the  offspring  of  human 
anatomy   and    physiology,    born   in   the   schools 
of  Vesalius,  Fabricius  ab  Aquapedente,  William 
Harvey   and    Malpighi.       Morphology,    on    the 
other   hand,    is   the    child    of    evolution,    reared 
under   the   tutelage    of    Cuvier    and    Lamarck, 
Von    Baer   and    Haeckel,    Darwin   and    Huxley, 
and    taken     into    the    service    of    comparative 
anatomy  and  embryology.      It    is   a   branch   of 
philosophy    dealing    partly   with    positive   data, 


2  THE   ART  OF   MORPHOLOGY 

partly  also  with  fanciful  abstractions  culled  from 
the  contemplation  of  concrete  phenomena,  but 
quite  distinct  from  the  soul  of  mankind,  although 
dependent  for  its  existence  upon  the  human 
understanding. 

Human  life  is  guarded  and  controlled  by  laws, 
the  principles  and  spirit  of  which  have  been 
evolved  out  of  chaos  by  the  mind  of  man 
stimulated  thereto  by  the  necessities  of  the 
fundamental  claims  of  property,  of  exogamy, 
and  of  fear.  The  existence  of  non-human  and 
undomesticated  organisms  is  governed  by 
natural  laws  which  regard  not  individuals  except 
in  so  far  as  they  are  subservient  to  the  needs 
of  the  race  to  which  they  belong.  In  civilisation 
the  individual  is  sacred  and  inviolable,  but  in 
nature  obviously  at  a  discount. 

The  geological  record,  made  up  as  it  is  of 
organic  remains  preserved  in  a  more  or  less 
fragmentary  condition,  must  always  continue  to 
be  incomplete  precisely  in  regard  to  those 
organisms  about  whose  past  history  information 
would  be  peculiarly  acceptable,  namely,  the  soft- 
bodied  animals ;  but  imperfect  though  it  be,  it 
suffices  to  show,  what  the  historical  record  of 
seven  thousand  years  fails  to  prove,  that  there 
is  a  specific  as  well  as  an  individual  longevity  ; 
and  that  when  the  form  manifested  by  a  given 
species,  the  resultant  of  a  particular  combination 
of  heritable  characters,  becomes  antiquated  and 


PURPORT  OF  LIFE  3 

incapable  of  prolonging  the  struggle  against 
the  influences,  whether  aggressively  hostile  or 
relentlessly  dominating,  which  possess  it  from 
within  and  from  without,  it  must  give  place,  by 
substitution,  to  a  new  combination  represented 
by  a  new  form,  which  may  be  an  outsider  or  a 
scion  of  the  old  stock. 

Perhaps  it  may  be  said  without  undue  pre- 
sumption that  it  is  one  of  the  vanities  of  the 
imagination  to  consider  that  everything  exists 
for  his  special  behoof.  This  is  one  of  those 
pious  beliefs  which  are  common  to  men  and 
dogs.  One  of  the  elementary  truths  revealed 
by  biology  is  this  :  that  whatever  use  man  may 
make  of  his  contemporaries,  and  however  success- 
ful he  may  be  in  bending  them  to  his  will  or  in 
exterminating  them,  the  fact  remains  that  they 
are  not  here  to  do  his  bidding  but  to  find  their 
own  sustenance,  to  secure  their  own  safety,  and 
to  propagate  their  own  kind.  They  have  an 
objective  existence  guided  by  instincts  of  which 
the  superficial  reactions  alone  can  be  observed 
and  named.  Biology  is  based  upon  a  detached 
investigation  of  the  properties  of  living  matter, 
and  is  thus  distinguished  from  teleology  which 
depends  upon  an  anthropocentric  interpretation 
of  the  organic  environment,  whereas  mythology 
may  be  said  to  proceed  from  an  anthropomorphic 
conception  of  the  inorganic  environment. 

Anatomy,  defined  as  the  science  of  the  structure 


4  THE  ART   OF  MORPHOLOGY 

of  organised  bodies,  is  usually  comprehended  in 
the  three  faculties  of  human  anatomy,  zoologi- 
cal anatomy  (zoology),  and  vegetable  anatomy 
(botany).  Each  of  these  primary  disciplines 
introduces  the  student  to  several  paths  of  learn- 
ing, e.g.,  topographical  anatomy  (anatomy  sensu 
stricto),  physiological  anatomy  (physiology  or  the 
treatment  of  structure  in  relation  to  function), 
histological  anatomy  (histology  or  microscopic 
anatomy),  ontogenetic  anatomy  (embryology), 
and  philosophical  anatomy  (morphology).  These 
are  what  Owen  (1866)  called  the  ways  of 
anatomy. 

Histology,  the  anatomy  of  the  constituent 
parts  or  tissues  of  organs,  is  investigated  by 
microscopical  and  microchemical  methods  which 
were  pioneered  by  Xavier  Bichat  whose  work, 
"  Anatomic  ge"nerale  appliquee  a  la  Physiologic  " 
(1801),  preceded  by  more  than  a  quarter  of  a 
century  the  enunciation  of  the  cell-theory.  This 
theory  laid  the  foundation  of  a  great  part  of 
modern  biological  research,  and  it  is  a  significant 
circumstance  that  it  was  published  in  the  decade 
which  witnessed  the  death  of  Cuvier  (1832). 
It  teaches  that  the  ultimate  units  of  all  the 
tissues  of  an  organism  consist  of  nucleated  proto- 
plasmic bodies,  conventionally  termed  cells,  which 
are  derived  by  repeated  cell  -  division  from  a 
primordial  fertilised  egg-cell ;  and  it  also  forms 
the  basis  of  the  subdivision  of  the  animal 


ECONOMIC  ZOOLOGY  5 

kingdom   into    protozoa    or    unicellular   animals, 
and  metazoa  or  multicellular  animals. 

Having  now  briefly  defined  the  objects  and 
the  position  of  our  enquiry,  let  us  turn  for  a 
moment  to  consider  it  from  the  economic  or 
cui  bono  standpoint.  Economic  zoology  is  to 
morphology  what  the  industrial  arts  are  to  fine 
art  —  with  a  difference  upon  which  it  is  not 
necessary  to  insist.  Many  of  the  aforesaid 
objects  of  enquiry  are  so  rarely  seen  that  their 
disappearance  from  the  face  of  the  earth  would 
not  produce  any  perceptible  effect  upon  the 
economy  or  balance  of  nature.  Most  living 
things  are  popularly  conceived  to  be  of  no  use 
in  the  wild  state ;  and  their  extermination,  so 
long  as  they  remain  indomitable  and  refuse,  as 
it  were,  to  take  the  oath  of  allegiance  to  man, 
appears  to  be  inseparable  from  the  progress 
of  civilisation.  In  many  cases  their  specific 
longevity  seems  to  be  approaching  the  end,  in 
precisely  the  same  manner  as  is  the  racial 
longevity  of  the  primitive  and  fast  vanishing 
races  of  mankind. 

The  Rev.  Gregor  Mendel,  Abbot  of  Brtinn, 
whose  experiments  in  practical  heredity  are  now 
appraised  by  some  of  his  followers  at  an  even 
higher  rate  than  those  of  Charles  Darwin,  was 
coeval  with  his  posthumous  rival,  though  appar- 
ently unknown  to  him  even  by  repute.  They 

were     contemporaries     working     independently 

A  2 


6  THE   ART  OF   MORPHOLOGY 

along  the  same  general  lines  though  with  differ- 
ent perspectives  and  methods,  the  one  exer- 
cising a  profound  substantive  influence  upon 
his  day  and  generation,  and  occasioning  a  great 
public  disputation,  the  personal  influence  being 
permanent,  the  public  disputation  ending  in 
platitudes ;  the  other,  in  the  quietude  of  the 
sanctuary,  unseen  and  unknown  except  to  a  few 
intimates,  pursuing  far-reaching  numerical  re- 
searches which  have  been  rescued  from  oblivion 
years  after  their  good  author  had  been  laid  to 
rest,  secure  alike  from  excommunication  and 
from  hair-splitting  controversy.  Students  of 
practical  questions  of  heredity  confess  to  the 
hope  that  their  investigations  may  lead  to  the 
regeneration  not  only  of  cultivated  and  domesti- 
cated races  of  plants  and  animals  but  also  of  the 
human  race,  by  the  selection  and  perpetuation  of 
desirable  characters  and  the  weeding  out  of  un- 
desirables. From  this  point  of  view  their  work 
has  undoubtedly  a  direct  economic  significance. 

In  the  same  way  as  benefiting  humanity,  the 
discoveries  in  protozoology  and  parasitology 
which  have  rendered  the  infancy  of  this  century 
so  illustrious,  of  the  relations  of  protozoa  to 
certain  epidemic  and  epizootic  diseases,  the  life- 
histories  and  nuclear  changes  of  the  parasites, 
and  their  transmission  by  intermediate  hosts ; 
as  well  as  such  discoveries  as  that  of  the 
conveyance  of  plague  bacilli  by  rat-fleas,  of  the 


SUMMUM  BONUM  7 

micrococci  of  Malta  fever  by  goats,  and  Looss's 
discovery  of  the  stereotropism  or  contact  require- 
ments of  the  larva  of  the  tunnel  worm  (Anchylo- 
stomum)  —  all  of  these  are  of  primary  hygienic 
importance. 

So  much  however  cannot  be  said  convincingly 
of  purely  morphological  researches.  The  origin 
of  the  human  race,  the  ancestry  of  vertebrates, 
the  correct  systematic  position  of  fleas,1  and 
other  cognate  problems  perennially  awaiting  solu- 
tion, are  matters  of  no  importance  in  domestic 
life,  and  it  is  hard  to  believe  that  they  ever 
will  possess  any  such  value.  Although  it  may 
be  true  that  to  the  individual  worker  it  is  the 
summum  bonum  when  his  science  overtakes  a 
human  need,  it  does  not  follow  that  we  must 
accept  the  theory  that  every  scientific  fact  will 
sooner  or  later  directly  and  materially  benefit 
mankind  at  large,  and  that  this  consideration 
should  be  the  guiding  motive  of  scientific 
investigation.  Many  precious  facts  have  been 

1  For  an  account  of  a  group  of  wingless  Diptera  to  which  fleas 
do  not  appear  to  be  related  except  by  convergence,  and  the 
amusing  controversy  to  which  it  gave  rise,  the  following  papers  may 
be  consulted  : — I.  F.  Dahl,  "  Puliciphora,  eine  neue,  flohahnliche 
Fliegengattung,"  Zool.  Anz.,  1897,  pp.  409-412  ;  translated  by  E.  E. 
Austen  in  Ann.  Nat.  Hist,  (vii.)  I.  pp.  99-101,  1898  (Puliciphora, 
a  new  flea-like  genus  of  Diptera.)  2.  B.  Wandolleck,  "1st  die 
Phylogenese  der  Aphanipteren  entdeckt?"  Zool.  Anz.,  1898,  pp. 
180-182.  3.  F.  Dahl,  "  Uber  Puliciphora  lucifera,"  Zool.  Anz.,  1898, 
p.  308.  4.  B.  Wandolleck,  "  Die  Stethopathidae  eine  neue  fliigel 
— und  schwingerlose  Familie  der  Diptera,"  Zool.  Jahrb.  Syst.,  xi., 
1898,  pp.  412-441. 


8  THE   ART  OF   MORPHOLOGY 

lost  for  ever  in  the  cataclysms  of  the  past 
without  damaging  the  prospects  of  the  human 
race.  It  is  certain,  however,  that  morphological 
researches,  and  the  speculations  which  proceed 
from  them,  contribute  to  the  precision  of  our 
rational  conceptions,  and  on  this  account  they 
cannot  be  neglected.  Their  intellectual  value  is 
an  indication  of  their  art.  But  I  will  not  run 
the  risk  of  sapping  the  strength  of  morphology 
by  enquiring  too  closely  where  it  lies. 

Biometrical   researches,    using  the   expression 
in   an   extended   sense    to  include  all  statistical 
studies   of  variation   and   cell-lineage,   have,   on 
the  whole,  as  little  in  common  with  morphology 
as  with  parasitology.     But  the  point  of  view  of 
biologists  undergoes  periodical  change,  as  indeed 
it   must   do,    for    unless    the    methods    and    the 
outlook   are  varied   the   art  will    be   lost.     The 
scope  of  any  method  is  limited,  more  narrowly 
in  some  cases  than  in  others ;  and  after  it  has 
been  tested  by  a  crowd  of  workers  its  limita- 
tions   become    uncomfortably    patent,    and    one 
turns  with  relief  to   something  altogether  new. 
At  the  same  time  it  should  be  remembered  that 
the  application  of  what  may  be  termed  ad  hoc 
methods    to    the    elucidation    of    morphological 
problems    may    only   serve   to    demonstrate   the 
inadequacy  of  such  methods  in  respect  of  that 
particular  application.     What  yields  good  results 
in   heredity   may   not   be   equally   fruitful  when 


TYPOLOGY  9 

applied  to  phylogeny,  the  origin  of  the  leading 
branches  of  the  tree  of  life. 

Morphology  in  general  treats  of  the  evolution 
of  forms  and  organs  within   the  compass  of  a 
given    type.       Phylogeny    does    this    too,    and 
more   besides,    since    it   is   concerned   with    the 
almost  impossible  task  of  tracing  the  pedigrees 
of    the    primary    types    themselves.       A    little 
explanation  may  be  necessary  at  this  point   to 
render    my    meaning    intelligible.       The    word 
"  type  "  has  been  used  in  many  different  senses, 
and  is  consequently  a  dangerous  equivocal  word 
to  trifle  with,  though  very  convenient.     Examples 
of   types   of   the   first   order   of   magnitude   are 
the   platyhelminth   type,   the   annelid    type,    the 
arthropod  type,   the  molluscan   type,   the  verte- 
brate   type.       Probably    in    consequence    of    a 
reaction   against   the   old    dogmatic   use   of   the 
term,  these  types  have,  at  one  time  or  another, 
been  connected  together  by  imaginary  links  in 
a  manner  which    in  many  cases,   is  now  recog- 
nised as  being  quite  inadmissible  or,  as  we  may 
otherwise   express  it,   in   a  manner   contrary  to 
the  canons  of  the  art  of  morphology.     Hopeless 
pictures  and  impossible  pedigrees  have  resulted 
from   this   confusion   of  ideas.     The   criteria   of 
homology  are  indeed  hard  to  define,  being  partly 
intuitive,  but  we  may  find  some  indications  which 
may  assist  towards  their  future  formulation. 
The  fish  type,  the  bird  type,  and  the  mammalian 


io  THE   ART   OF   MORPHOLOGY 

type  may  be  quoted  as  instances  of  the  second 
order  of  magnitude ;  the  hoofed  mammal  or 
ungulate  type  and  the  primate  type  are  of  the 
third  order ;  the  odd-toed  ungulate  of  the  fourth 
order ;  the  equine  type  and  the  human  type  are 
examples  of  the  fifth  order  of  magnitude.  Lower 
categories  are  exemplified  by  the  type  genus  of 
a  family,  the  type  species  of  a  genus,  and  the 
type  individual  of  a  species,  the  last  being  purely 
conventional,  referring  to  the  actual  specimen 
upon  which  the  original  specific  description  was 
based.  The  types  of  cultivated  races  are  deter- 
mined in  a  somewhat  different  manner. 

From  the  fourth  grade  downwards  it  is 
frequently  possible  to  determine  the  pedigree 
of  a  specialised  type  from  a  more  generalised 
ancestry,  with  considerable  probability  of  accuracy 
and,  in  some  cases,  as  with  the  pig,  horse,  and 
elephant,  with  a  great  deal  of  precision.  In  the 
superior  grades  it  becomes  increasingly  difficult 
to  arrive  at  an  understanding.  It  is  something 
to  be  able  to  say  that  the  vertebrate  type  can 
be  traced  with  certainty  to  a  chordate  type ; 
but  it  cannot  be  asserted  without  contradiction 
that  the  craniate  vertebrate  is  derived  from  an 
acraniate  chordate ;  because  the  gap  between 
these  two  grades  of  organisation  is  too  great, 
and  many  of  the  intervening  stages  of  substitu- 
tion and  of  cephalisation  (head  -  formation)  are 
lost. 


ORGANIC   SYMMETRY  n 

Phylogeny  and  heredity  illustrate  different 
phases  of  the  theory  of  descent  with  modifica- 
tion, and  have  to  be  approached  by  appropriate 
methods.  The  test  of  breeding,  which  is  crucial 
in  heredity,  is  obviously  inapplicable  in  a  phylo- 
genetic  investigation  where  the  last  positive  links 
in  the  chain  of  evidence  can,  as  a  rule,  only  be 
afforded,  if  at  all,  by  palaeontology. 

Morphology,  in  its  aesthetic  aspect,  is  the 
perception  of  symmetry  and  organic  beauty.  Its 
interest  and  importance  lie  less  in  its  ultimate 
truth  than  in  its  relative  completeness,  by  which 
I  mean  its  capacity  for  embracing  the  widest 
possible  range  of  facts  without  ignoring  those 
which  may  not  fall  into  harmony  with  a  parti- 
cular theory. 

The  distinction  between  convergent  and  normal 
morphogeny  has  long  pervaded  biological  litera- 
ture ;  but  I  believe  it  to  be  a  fact  that  the 
equality  of  interest  which  attaches  to  the  two 
branches  of  the  subject  has  not  been  recognised 
in  an  adequate  manner  hitherto.  It  will  appear 
in  the  course  of  this  essay  that  convergence  is 
neither  identical  with  homoplasy l  nor  with  ceno- 
genesis,2  but  that  it  includes  these  and  some- 


1  Homoplasy  signifies    similarity  of   form   unaccompanied  by 
community  of  pedigree.     As  explained  below,  we  owe  the  term 
to  Sir  Ray  Lankester. 

2  Cenogenesis    implies    the    origin    of   structural   features    by 
relatively  recent    adaptation,   in    contrast    with    palingenesis  or 
primordial  adaptation.     The  terms  are  due  to  Professor  Haeckel. 


12  THE  ART  OF   MORPHOLOGY 

thing  else  besides.  All  homoplasy  is  convergence, 
but  all  convergence  is  not  homoplasy ;  and  the 
same  dictum  may  be  repeated,  mutatis  miitandis, 
for  cenogenesis. 

Professor  H.  F.  Osborn,  in  the  paper  to  which 
I  have  referred  in  the  Preface,  finds  homoplasy 
to  be  of  very  great  importance  in  the  evolution 
of  mammalian  teeth,  "  because  it  seems  to  coin- 
cide with  the  principle  of  definite  or  determinate 
evolution  "  which  has  proceeded  "  independently 
in  a  great  many  different  families  of  mammals." 
He  discusses  the  special  value  of  the  evolution 
of  teeth  as  bearing  upon  homoplastic  mutations, 
inasmuch  as  teeth  are  preformed  beneath  the 
gum  so  that  "new  cusps,  folds,  crests,  and  styles 
are  invariably  congenital." 


CHAPTER   II 

PHYSIOLOGICAL   CLASSIFICATION 

THE  animal  kingdom,  according  to  the  teaching 
which  was  customary  in  Professor  Huxley's  time, 
was  arranged  under  the  two  great  divisions  of 
the  vertebrata  or  animals  provided  with  a  jointed 
axial  skeleton,  the  backbone,  and  the  invertebrata 
or  animals  destitute  of  a  backbone;  whereby  a 
single  phylum  or  primary  branch  of  the  tree  of 
life  was  contrasted  and  placed  upon  an  apparent 
equality  with  a  whole  complex  of  other  phyla. 
This  method  of  classification  denoted  one  of 
the  great  generalisations  in  zoology  at  the  birth 
of  the  nineteenth  century,  enduring  from  the 
beginning  to  the  end  of  that  century.  Those 
who  attend  to  these  matters  no  longer  continue 
to  oppose  the  homogeneous  group  of  the  verte- 
brates to  the  negative  association  of  invertebrates. 
The  former  term,  introduced  by  Lamarck  and 
Cuvier,  retains  its  full  significance,  but  the  word 
invertebrate,  though  still  useful  as  an  adjective, 
is  comparatively  barren  as  an  idea. 

The  classification  now  adopted  is  one  based 

13 


i4  PHYSIOLOGICAL   CLASSIFICATION 

upon  a  profounder  analysis  of  the  animated 
frame,  namely,  upon  the  cellular  constitution  of 
the  body.  This  second  great  generalisation,  as 
we  have  seen  above,  had  already  been  achieved 
during  the  first  half  of  the  preceding  century, 
but  the  fate  of  the  backbone  in  classification 
was  not  sealed  until  the  end.  In  the  light  of 
recent  researches  we  may  find  a  further  reason, 
besides  the  fallacy  of  contrasting  non-equivalent 
groups,  for  discarding  the  older  method.  The 
presence  of  a  backbone  is  a  character  which 
does  not  pair  with  the  absence  of  one,  but  it 
does  pair  with  the  presence  of  a  notochord. 

We  may  venture  to  lay  down,  as  a  funda- 
mental principle  in  morphology,  that  only  those 
characters  pair  which  are  connected  together  by 
the  bond  of  substitution.  By  way  of  extending 
this  principle  one  step  further  whilst  reference 
is  being  made  to  the  vertebrata,  we  may  add 
that  the  presence  of  a  bony  endoskeleton  is  a 
character  which  pairs  morphologically  with  a 
cartilaginous  endoskeleton,  but  not  with  a  horny 
exoskeleton.  On  the  other  hand  the  vertebrate 
endoskeleton,  in  its  capacity  of  affording  attach- 
ment to  muscles,  can  be  compared  physiologically 
with  an  arthropod  exoskeleton  or  a  molluscan 
shell. 

A  morphological  classification  must  be  physio- 
logically true,  i.e.,  it  must  conform  to  physio- 
logical laws  ;  but  a  physiological  classification  is 


DUAL   CLASSIFICATION  15 

under  no  corresponding  obligation,  and  it  is 
partly  through  losing  sight  of  this  circumstance 
of  the  differential  purport  of  morphology  and 
physiology,  partly  also  through  lack  of  know- 
ledge of  palaeozoic  physiology,  that  much  con- 
fusion has  arisen. 

Returning  now  to  the  consideration  of  the 
dual  classification  of  the  animal  kingdom : — the 
current  system,  which  has  received  so  much 
additional  stability  by  the  recent  progress  of 
protozoology,  recognises  the  two  great  sub- 
divisions of  the  protozoa  and  the  metazoa.  It 
is  sometimes  considered  an  advantage  to  unite 
all  unicellular  organisms  under  the  Haeckelian 
term  protista  from  which  are  derived  the  parallel 
stems  of  the  metazoa  and  the  metaphyta  (multi- 
cellular  plants).  In  the  vegetable  kingdom  the 
transition  from  unicellular  to  multicellular  forms 
is  graduated  even  amongst  existing  natural 
orders ;  and  the  algae  include  both  kinds. 

The  protozoa  are  commonly  treated  as  a 
phylum  of  the  animal  kingdom  equivalent  in 
classification  to  the  other  phyla,  but  this  pro- 
cedure is  not  in  strict  accordance  with  facts. 
The  protozoa  occupy  a  position  which  is  unique, 
and  they  are  rightly  contrasted  with  the  poly- 
phyletic  metazoa.  There  is  no  such  abstraction 
as  a  protozoan  type  in  the  same  sense  as  there 
is  a  molluscan  type  or  ,a  vertebrate  type, 
because  the  combination  of  characters  which  is 


16  PHYSIOLOGICAL    CLASSIFICATION 

common  to  all  protozoa  is  also  that  which  is 
common  to  all  metazoan  cells,  except  their  in- 
dependence. The  positive  distinction  between 
protozoa  and  metazoa  is  thus  justified  alike  on 
morphological  and  on  physiological  grounds. 

It  remains  to  be  seen  whether  there  are  not 
some  other  properties  of  animals  that  could  be 
utilised  temporarily  for  the  purpose  of  obtaining 
a   comprehensive    survey,    beyond    those    which 
are    afforded   by   their   bodily   composition.      A 
grouping    of    physiologically   comparable    forms 
may    indeed   be    very    instructive,    and    it    has 
this  additional  advantage  that  it  enables  us  to 
visualise,    in    a    single    perspective,   a   multitude 
of  facts  which  would  otherwise  lie  beyond  our 
range  for  the  time  being.     It  will  thus  be  seen 
that    physiology   can    render    great    service    in 
marshalling    together     scattered     morphological 
units.       In   support   of  the   soundness   of  these 
assertions  I  may  quote  the  following  statement 
from  Prof.  A.  A.  W.   Hubrecht's  recent  mono- 
graphic    essay     on     the     "  Early     Ontogenetic 
Phenomena   in    Mammals."1     Referring    to   the 
relation   between    trophoblast2    and   amnion    of 
which  he  is  the  exponent,  he  says  : — 

"A.  Willey  ('98)  has  speculated  upon  similar 
relations  between  arthropod  embryos  and  their 
larval  envelope,  also  designated  as  amnion,  in 

1  Quart.  Journ.  Micro.  So.,  vol.  liii.,  1908,  p.  76. 

2  The  trophoblast  is  the  outer  absorbent  germ-layer  attached  to 
the  embryos  of  many  viviparous  animals. 


LAND   AND  WATER  17 

Peripatus,  Lepisma,  Gryllus,  Forficula,  and 
others ;  has  rightly  interpreted  the  direct  com- 
parability with  the  vertebrate  trophoblast,  and 
has  looked  upon  it  (as  I  have  done  ['95  B]  for 
vertebrates)  as  an  adaptation  to  a  viviparous 
habit  acquired  by  the  terrestrial  descendant  of 
an  aquatic  ancestor."  . 

An  example  of  a  simple  and  obvious  physio- 
logical system  of  classification,  based  upon  dis- 
tribution and  covering  the  entire  animal  kingdom, 
is  that  which  distinguishes  between  aquatic  and 
land  animals.  There  is  a  peculiar  propriety  in 
this  disposition  inasmuch  as  all  the  groups  of 
land  animals  have  had  a  more  or  less  remote 
aquatic  ancestry,  some  inconceivably  remote, 
others  comparatively  recent ;  whilst  others  again 
have  reverted  to  an  aqueous  medium  in  which 
their  lives  are  spent.  Land  mollusca,  land 
leeches,  land  nemertines,  land  planarians,  and 
land  crabs,  are  examples  of  terrestrial  groups 
which  have  had  a  recent  aquatic  ancestry,  their 
immediate  congeners  being  still  aquatic  animals. 

The  general  structural  differences  which  are 
associated  with  the  radical  change  of  environment 
are  chiefly  connected  with  the  organs  of  respira- 
tion and,  to  a  less  extent,  those  of  locomotion. 
In  some  cases,  as  with  the  operculate  land 
molluscs,  the  change  from  water  to  land  has 
only  affected  the  organ  of  respiration.  In  others 
no  striking  alteration  is  manifested  in  essential 
organs,  although  accessory  structures  sometimes 

B 


i8  PHYSIOLOGICAL  CLASSIFICATION 

appear,  as  in  Birgus  latro,  the  coconut  robber 
crab,  where  there  is  an  accessory  pulmonary 
chamber  above  the  gills,  and  in  Ampullaria,  a 
tropical  freshwater  operculate  snail  living  in  tanks 
and  swamps  exposed  to  drought,  where  there 
is  an  analogous  arrangement.1  An  interesting 
and  rather  subtle  change  has  taken  place  in  the 
habits  of  typical  land  leeches  in  that  they  have 
lost  the  power  of  swimming,  having  become 
entirely  stereotropic. 

Purely  aquatic  creatures  are  those  which 
breathe  by  absorbing  the  oxygen  dissolved  in 
the  water,  and  they  are  sometimes  called  "  water- 
breathing  "  animals  on  that  account.  If  kept  in 
a  limited  volume  of  water  the  oxygen  would 
become  exhausted  and  the  animals  asphyxiated 
unless  the  water  was  frequently  renewed  as  it 
is  in  an  aquarium.  Asphyxiation  sometimes 
takes  place  under  natural  conditions  in  low-lying 
swamps  when  the  water  in  the  deeper  places 
becomes  stagnant  after  a  prolonged  drought. 
The  mud-dwelling  fishes  at  the  bottom  may  then 
be  seen  swimming  spasmodically  towards  the 
surface  to  gulp  in  atmospheric  air,  often  too 
late  to  recover  from  the  poisonous  effect  of  the 
noxious  gases  below. 

There  are  many  partially  or  completely  aquatic 
animals  which  find  their  food  under  water  and 


1  Birgus  and  Ampullaria  are  figured  in  Semper's  "  Animal  Life,5' 
London,  1890. 


AMPHIBIA  19 

whose  appendages  are  adapted  for  swimming, 
although  they  breathe  atmospheric  air  and  are 
therefore  true  air-breathers.  The  cetacea  and 
the  sirenia  are  completely  aquatic  in  so  far  that 
if  they  are  driven  ashore  by  storm  or  accident 
they  are  irretrievably  lost ;  at  the  same  time 
they  can  only  breathe  air.  The  distinction 
between  air  -  breathers  and  water  -  breathers  is 
therefore  more  fundamental  than  that  between 
aquatic  and  terrestrial  animals.  The  inter- 
mediaries between  these  two  groups  are  the 
amphibious  creatures  which  are  by  no  means 
confined  to  the  class  Batrachia  or  Amphibia 
sensu  stricto,  but  include  all  land  crabs,  some 
molluscs,  and  even  certain  fishes,  both  dipnoan 
and  teleostean,  as  well  as  some  reptiles. 

The  essential  organs  which  perform  the 
principal  functions  of  the  body  might  each  in 
turn  be  made  the  basis  of  a  physiological  classifi- 
cation which  would  convey  a  certain  amount  of 
information  to  those  for  whom  such  knowledge 
possesses  any  value.  More  interesting  for  our 
present  purpose  are  such  groupings  as  depend 
upon  the  relations  of  animals  to  their  immediate 
surroundings  irrespective  of  the  actual  nature  of 
the  environment,  and  it  will  be  seen  that  the 
pairs  of  characters  illustrated  by  the  dual  sub- 
divisions which  will  be  discussed  below,  virtually 
coincide  in  their  nature  with  some  of  the  primary 
differences  between  animals  and  plants. 


20  PHYSIOLOGICAL  CLASSIFICATION 

The  various  tactics  to  which  animals  and  plants 
resort  when  driven  by  necessity  or  threatened  by 
danger,  and  the  reactions  which  they  exhibit 
towards  external  stimuli  such  as  heat,  light, 
drought,  moisture,  contact,  proximity  of  food 
and  of  hostile  influences,  have  long  engaged 
the  attention  of  biologists  for  the  reason,  amongst 
others,  that  the  differently  constituted  nervous 
system  of  invertebrate  animals  and  the  lack  of 
this  system  in  plants,  in  protozoa,  and  in  sponges, 
impart  a  special  character  to  their  vital  reactions, 
in  contrast  with  the  more  or  less  intelligent 
responses  of  craniate  vertebrates.1  All  the 
tendencies  or  "tropisms"  to  which  protoplasm 
is  liable  are  to  be  observed,  either  positively  or 
negatively,  in  every  organism,  and  frequently, 
at  different  times,  both  positively  and  negatively 
in  the  same  species. 

One  of  the  most  commonly  observed  reactions 
is  that  known  as  phototaxis  or  heliotropism,  these 
terms  being,  however,  not  quite  synonymous 
inasmuch  as  the  reaction  to  sunlight  (i.e.,  helio- 
tropism in  the  strict  sense)  is  not  the  same 

1  Cf.  J.  von  Uexkiill,  "  Leitfaden  in  das  Studium  der  experi- 
mentellen  Biologic  der  Wassertiere."  Wiesbaden,  1905. 

M.  Verworn,  "Allgemeine  Physiologic,"  4th  edit.,  Jena,  1903. 
English  translation  "  General  Physiology "  from  the  2nd  edit,  by 
F.  S.  Lee,  London,  1899. 

H.  S.  Jennings,  "  Behaviour  of  the  Lower  Organisms."  New 
York,  1906. 

J.  E.  Duerden,  "  On  the  Habits  and  Reactions  of  Crabs  bearing 
Actinians  in  their  Claws."  P.  Zool.  Soc.  London,  ii.  1905  (1906). 


ATTRACTION   OF   LIGHT  21 

as  that  to  artificial  light  (i.e.,  phototaxis  in 
the  narrower  sense).  Professor  Max  Verworn, 
whose  work  on  General  Physiology  has  done 
so  much  to  give  precision  to  the  interpretation 
of  elementary  vital  phenomena,  apparently  wishes 
to  discard  the  term  heliotropism  altogether  in 
favour  of  phototaxis ;  but  there  are  certain  cases 
which  oppose  this  blending  of  terms. 

The  nocturnal  lepidoptera  are  negatively 
heliotropic  but  positively  phototactic,  being  im- 
pelled by  an  irresistible  attraction  to  a  bright 
lamp.  Amphioxus,  the  lancelet,  is  passively 
heliotropic,  but  actively  phototactic,  evincing 
extreme  agitation  upon  the  approach  of  a  lighted 
candle  at  night.  In  the  same  way  fishes  are 
attracted  by  torchlight  at  sea,  and  lighted  lamps 
are  placed  at  the  head  of  fish-traps,  where  they 
are  kept  burning  all  night  to  attract  prawns 
which  are  required  for  bait  in  large  quantities. 
Lighted  prawn-traps  and  moth-traps  represent 
practical  applications  of  the  knowledge  of  the 
phototactic  properties  of  these  animals.  The 
phosphorescent  organs  of  deep-sea  fishes  no 
doubt  also  act  as  luminous  lures. 

An  intimate  acquaintance  with  the  habits 
and  reactions  of  animals  is  not  confined  to  the 
human  race.  Many  reptiles,  birds,  and  mammals 
are  accomplished  field  entomologists,  know- 
ing where  to  look  for  their  food,  and  how  to 
distinguish  edible  from  inedible  kinds.  The 

B  2 


22  PHYSIOLOGICAL   CLASSIFICATION 

tropical  tree  lizard  Calotes  will  take  its  stand 
beside  a  beehive  to  catch  the  bees  as  they  issue, 
swallowing  them  entire ; *  or  beside  an  incipient 
termite  mound  picking  off  the  workers.  Bulbuls 
will  gather  round  a  swarming  termite  nest  and 
decimate  the  winged  emigrants.  Bats  will  fly  to 
a  bright  light,  not  because  they  are  themselves 
attracted  by  it,  but  because  they  know  where  to 
find  insects,  especially  winged  termites,  which  are. 
Geckoes  and  toads  similarly  approach  artificial 
light  in  search  of  insect  food.  In  the  three 
last-mentioned  examples  of  nocturnal  animals 
seeking  their  food  by  artificial  light  we  find 
once  more  (as  in  the  case  of  moths)  positive 
phototaxis  in  apparent  conflict  with  negative 
heliotropism. 

It  seems  important  to  take  notice  of  this 
association  of  contrary  reflexes,  of  which  more 
instances  will  be  found  below.  Meanwhile  it 
may  be  useful  to  distinguish  habits  from  re- 
actions : — a  habit  is  the  behaviour  of  an  animal 
in  the  open ;  a  reaction  is  its  behaviour  in 
the  laboratory.  Finally,  whilst  dealing  with 
definitions  we  may  add  that  the  word  tropism 
means  the  tendency  to  react  in  a  definite  manner 
towards  external  stimuli. 

1  Mr  Francis  Buckland  has  recorded  a  similar  observation  of  the 
common  toad  eating  bees  in  Oxfordshire. 


CHAPTER    III 

EXPOSED   AND    CONCEALED   ANIMALS 
(PHANEROZOA    AND    CRYPTOZOA) 

PHANEROZOIC  or  diurnal  animals  are  positively 
heliotropic ;  cryptozoic  animals  including  crepu- 
scular, nocturnal,  and  subterranean  forms,  in 
fact  all  that  avoid  the  light  of  day,  are  negatively 
heliotropic.  Flights  of  butterflies  at  high  noon 
are  followed  by  swarms  of  hawk-moths  and 
noctuids  at  twilight.  The  contrast  is  particularly 
well  illustrated  on  a  broad  and  spectacular  scale 
by  the  interchange  of  amenities  between  such 
gregarious  creatures  as  crows  and  the  so-called 
flying  foxes  or  fruit-eating  bats,  due  to  their 
homing  in  the  same  place,  as  may  be  observed 
daily  in  certain  favourable  localities  in  the  mari- 
time districts  of  Ceylon.  I  recorded  these  facts 
in  "  Spolia  Zeylanica,"  after  mentioning  them 
in  a  letter  to  the  late  Professor  Alfred  Newton 
who  expressed  his  appreciation  of  them.  These 
animals  keep  remarkably  regular  hours  of  work 
and  sleep,  the  birds  foraging  by  day,  the  bats 
by  night. 

23 


24         EXPOSED  AND   CONCEALED  ANIMALS 

At  one  place  in  particular,  a  small  lighthouse 
islet  off  the  south-west  coast  named  Barberyn, 
which  is  covered  by  a  coconut  plantation,  they 
congregate  in  the  palm  trees  alternately  by  night 
and  by  day.  If  one  crosses  over  to  the  island 
in  the  heat  of  the  day  all  is  quiet  and  nothing 
out  of  the  common  is  to  be  noted ;  but  about 
the  time  of  sunset  a  great  commotion  of  crows l 
among  the  tree  tops  bursts  upon  the  ear,  and 
gradually  subsides  in  the  dusk  of  the  evening. 
This  signalises  the  arrival  home  of  the  colony 
of  crows  after  their  day's  work  is  over.  The 
approach  of  sunrise,  on  the  other  hand,  is 
announced  by  the  chattering  and  squabbling 
of  numerous  flying  foxes2  overhead. 

At  sundown  the  passage  of  immense  flocks 
of  crows  and  flying  foxes  in  opposite  directions 
across  the  strait  which  divides  the  island  from 
the  mainland  can  be  witnessed,  the  former 
bound  for  the  island  to  rest  for  the  night,  the 
latter  speeding  their  way  to  the  mainland  intent 
upon  their  nocturnal  forage.  The  flying  foxes 
travel  on  the  average  distinctly  higher  than 
the  crows,  starting  singly  and  increasing  to 
large  flocks  of  twenty-five  and  upwards,  finally 
becoming  a  continuous  stream.  The  crows 
obviously  outnumber  the  bats,  although  weight 
for  weight  they  probably  represent  an  equiva- 

1  The  grey-necked  Indian  crow,  Coruus  splendens 

2  Pteropus  medius. 


CROWS   AND   FLYING   FOXES  25 

lent  bulk  of  living  matter.     In  the  evening  the 

crows  begin  to  arrive  in  small  numbers  before 

the  vanguard  of  the  bats  has  started,  increasing 

in  their  turn  to  large  battalions  until  a  period  of 

maximum  migration  is  reached,  and  then  troops 

of  bats  are  to  be  seen  passing  over  still  larger 

columns  of  crows  in  the  opposite  direction,  the 

whole   of   the    cross-migration    occupying   about 

half  an   hour.      The    reverse   passage,    namely, 

the  matutinal  flight,  takes  place  towards  sunrise, 

the   bats    returning    from    the  mainland   to   rest 

for  the  day  suspended  in  rows  from  the  midribs 

of  the  palm  leaves,  the  crows  crossing  over  on 

their    daily    quest    for   garbage.      This    instance 

may  be   classed   as  one  of  convergent  homing, 

the    same  trees  affording  hospitality  in   regular 

alternation  to  day-flying  birds  and  night-flying 

mammals. 

Phanerozoa  and  cryptozoa  are  two  well-marked 
physiological  groups,  comprising  between  them 
all  the  forms  of  animal  life  ;  and  the  distinction 
is  paralleled  by  the  following  more  fundamental 
contrast  between  animals  and  plants.  In  general 
habit  and  mechanism  of  growth  the  vegetable 
kingdom  as  a  whole  exhibits  positive  phototaxis 
in  the  wide  sense,  whereas  the  animal  kingdom 
as  a  whole  seems  to  show  negative  phototaxis ; 
the  former  proposition  can  be  accepted  without 
question,  but  the  latter  requires  substantiation. 
Passing  over  the  vast  array  of  strictly  cryptozoic 


26         EXPOSED  AND  CONCEALED   ANIMALS 

forms,  it  may  be  noted  in  the  first  place  that 
while  the  green  colouring  matter  of  plants 
effects  its  characteristic  reaction,  the  chlorophyll 
reaction,  by  reason  of  its  exposure  to  sunlight, 
the  respiratory  pigments  of  animals  which  pro- 
mote another  kind  of  gaseous  interchange,  are 
enclosed  in  blood-vessels  and  sinuses,  generally 
concealed  from  the  light  by  an  opaque  skin,  and 
always  independent  of  the  light,  even  though  the 
skin  should  happen  to  be  transparent. 

As  a  second  example  to  illustrate  broadly  the 
essential  cryptotaxis  (tendency  to  concealment) 
of  animals,  may  be  mentioned  the  well-known 
fact  that  even  among  the  higher  vertebrates 
many  birds  and  most  mammals  are,  in  the  wild 
state,  concealed  from  view  by  their  protective 
coloration.  Of  course  this  does  not  in  itself 
shelter  them  from  the  sun ;  the  environment 
provides  the  cover ;  but  it  protects  them  from 
the  visual  acuity  of  their  enemies,  many  of 
which  are  notoriously  cryptotactic.  A  common 
method  of  shooting  wild  animals,  for  sport  or 
for  food,  is  to  lie  in  wait  at  night  concealed 
behind  an  ambush  within  range  of  a  water-hole 
where  they  will  come  to  drink.  The  largest 
mammals,  whether  carnivorous  or  herbivorous, 
such  as  the  tiger,  elephant,  and  giraffe,  are 
known  to  be  almost  invisible  when  at  rest  in 
the  forest,  jungle,  and  high  grass  which  they 
frequent  in  the  daytime.  In  Ceylon,  the  rounded 


CHROMATISM  37 

bosses  of  gneiss  with  their  dark  smoothly 
weathered  surfaces,  which  are  such  prominent 
features  in  the  landscape  in  many  parts  of  the 
country,  often  present  an  uncommon  resemblance 
to  the  form  of  the  elephant ;  and  the  converse 
is  equally  true  as  may  be  verified  when  a  herd 
of  elephants  is  seen  against  a  background  of 
gneiss,  the  likeness  being  based  upon  the  posses- 
sion of  a  common  ground  colour  superimposed 
upon  a  ponderous  bulk. 

Gorgeously  coloured  butterflies  and  beetles, 
small  flower-haunting  birds,  such  as  the  humming- 
birds of  the  New  World  and  the  sun-birds  of  the 
Old  World,  whose  plumage  reflects  a  wonderful 
metallic  lustre,  larger  birds  with  majestic  males, 
the  powerful  diurnal  birds  of  prey,  and  others 
afford,  to  the  general  rule  of  concealment, 
exceptions  which  do  not  stultify  the  rule  but 
are  explicable  on  special  grounds  of  adaptation, 
selection,  season,  locality,  and  dominance. 

Butterflies  exhibit  themselves  in  their  seasonal 
flights  sometimes  in  vast  numbers,  but  their 
abundance  at  certain  times  only  serves  to 
accentuate  their  absence  or  rarity  at  others ;  the 
rapidity  with  which  an  immense  noon-day  swarm 
can  efface  itself  is  marvellous  to  behold ;  while 
the  pupae  and  even  the  larvae  are  often  quite 
concealed  by  various  devices  of  shape,  position, 
and  colour.  So  with  diurnal  birds,  the  nests  of 
the  commonest  are  often  very  hard  to  find  and 
rarely  seen. 


28         EXPOSED   AND    CONCEALED   ANIMALS 

Land  planarians  offer  instances  of  cryptozoic 
animals  which  are  brightly  coloured ;  the  species 
are  differentiated  according  to  their  colour  mark- 
ings, and  richly  tinted  individuals  are  to  be  found 
in  rotten  logs  in  a  dormant  state  by  day,  with  the 
body  looped  in  a  characteristic  manner;  if  left  un- 
observed for  a  short  time  exposed  to  light,  they 
glide  rapidly  away  to  a  place  of  concealment. 

A  journey  through  any  tropical  forest  or 
jungle,  or  even  a  little  reflection  will,  I  think, 
suffice  to  convince  one  that  while  the  vegetation 
is  luxuriantly  phanerotactic,  animal  life  is  pre- 
dominantly cryptotactic.  The  jungle  is  like  the 
desert  and  the  ocean,  to  all  superficial  appear- 
ances frequently  devoid  of  animal  life.  This  is 
possibly  not  the  impression  which  one  would 
receive  from  the  perusal  of  faunistic  works ;  but 
it  is  certainly  that  which  is  produced  by  observa- 
tions in  the  open,  and  I  regard  it  as  one  of  the 
radical  bionomical  or  habitual  differences  between 
animals  and  plants. 

Vegetation  is  always  exposed,  it  is  permanently 
phanerotactic,  but  the  most  conspicuous  animals 
in  the  natural  state  are  only  partially  or  periodi- 
cally so.  In  many  instances,  especially  among 
the  hoofed  mammals  (Ungulata),  the  cryptotaxis 
can  be  properly  appreciated  only  under  natural 
conditions.  Arboreal  mammals,  such  as  squirrels 
and  monkeys,  are  amongst  the  most  clearly  phan- 
erotactic members  of  the  class  Mammalia,  but 


DAY   AND   NIGHT  29 

many  of  the  arboreal  Lemurs,  the  most  primitive 
of  the  order  Primates,  are  strictly  nocturnal  and 
hence  cryptotactic.  Burrowing  mammals  are 
generally  nocturnal  as  well,  including  the  egg- 
laying  mammals  of  Australia,  the  scaly  ant-eaters, 
ground-porcupines,  and  others  too  numerous  to 
mention  ;  the  combination  of  fossorial  with  diurnal 
habits  is  rare,  the  common  rabbit  affording  an 
instance  of  it.  Others  resort  during  the  daytime 
to  ready-made  holes,  hollow  trees  and  caves,  as 
the  dasyures,  mungooses,  and  sloth-bears.  Some 
species  exhibit  mixed  habits  ;  but  the  cryptotactic 
tendency  is  universal  amongst  mammals. 

It  is  not  possible  to  tabulate  lists  of  phaner- 
ozoa  and  cryptozoa,  because  all  animals  have 
cryptozoic  potentialities.  Those  that  are  not 
definitely  cryptotactic  have  the  power  of  retreat- 
ing to  cover  at  the  sign  of  danger ;  or  of  causing 
the  aggressor  to  vanish  from  the  scene  by  reason 
of  their  weapons  of  offence.  The  scent-glands 
of  the  mephitic  skunk,  of  the  naked  bat  (Chiro- 
meles],  and  in  a  lesser  degree  of  the  civet  cats, 
are  cryptotactic  organs  of  a  virulent  type  not- 
withstanding that  they  effect  their  object  through 
the  olfactory  sense ;  besides  being  protected  by 
their  secretions,  these  animals  are  nocturnal. 
Scorpions,  centipedes,  and  poisonous  snakes  are 
cryptozoic  as  well  as  armed  for  attack,  so  that 
they  also  are  doubly  protected  and  doubly 
dreaded  by  their  enemies. 


30          EXPOSED   AND   CONCEALED   ANIMALS 

A  special  example  of  the  cryptozoic  tendency 
in  poisonous  snakes  is  the  behaviour  of  the  cobra 
in  captivity  in  the  presence  of  its  food.  Many 
Eastern  jugglers  and  gipsies  carry  cobras  about 
with  them  coiled  up  in  round  flat  baskets,  which 
are  kept  covered  except  during  a  performance. 
These  cobras  are  fed  upon  eggs,  which  they 
swallow  whole  without  breaking  the  shell.  If 
they  are  preparing  to  cast  their  skin,  and  the 
eyes  are  glazed,  they  do  not  take  food.  But 
if  they  are  in  a  normally  hungry  condition  they 
will  not  touch  a  proffered  egg  so  long  as  the 
basket  is  uncovered.  If  the  cover  is  removed 
again  two  or  three  minutes  after  it  has  been 
replaced  with  the  egg  in  the  basket,  the  egg 
will  be  found  to  have  been  swallowed,  and  can 
be  perceived  travelling  down  the  gullet  by  the 
protuberance  which  it  causes ;  a  second  egg 
may  be  taken  in  like  manner  shortly  afterwards. 

Mr  Frank  Buckland  ("  Curiosities  of  Natural 
History,"  second  series,  reprinted,  London  1903, 
p.  131)  wanted  to  see  whether  a  hedgehog  would 
eat  a  common  harmless  snake.  He  caught  a 
snake  near  Harrow,  and  bought  a  hedgehog 
in  St  Giles's.  For  several  mornings  he  placed 
them  together  on  the  grass  ;  but  they  took  no 
notice  of  each  other.  At  last,  one  evening,  he 
shut  them  both  up  in  a  box  together.  During 
the  night  the  hedgehog  attacked  and  devoured 
half  the  snake,  beginning  at  the  tail.  In  a 


PHOTOPHOBIA  31 

few  more  hours  the  rest  of  the  snake  was  con- 
sumed. 

Blood  -  sucking  leeches  prefer  to  feed  under 
cover  and,  when  gorged,  retreat  to  cover  as 
quickly  as  may  be.  A  Ceylon  land  leech  which 
I  was  keeping  under  observation  remained  on 
my  arm  for  ten  minutes  without  attempting  to 
puncture  the  skin,  in  a  contracted,  flushed, 
quiescent  posture.  I  then  placed  a  silk  handker- 
chief over  the  waiting  worm,  with  the  result  that 
two  minutes  later  a  puncture  was  felt,  and  the 
attachment  continued  for  thirty  -  eight  minutes. 
When  the  leech  was  placed  upon  a  dead  leaf  after 
the  meal  it  immediately  crawled  underneath. 

Apart  from  parasitism,  the  purest  expression 
of  cryptotaxis  is  that  which  is  manifested  by 
genuine  cave-dwellers,  and  by  animals  which  not 
only  burrow  underground,  but  also  find  their 
food  underground,  such  as  the  mole  -  crickets 
(Gryllotalpa1},  the  earthworms  and  the  creatures 
which  feed  upon  them,  namely,  moles,  worm-eat- 
ing slugs  (Testacella),  earth-snakes  (Uropeltid&\ 
and  apodous  Batrachia  (Cceciliidcz).  I  have  kept 
an  Ichthyophis  alive  for  ten  months  in  one  half 
of  a  coconut  shell  covered  over  by  the  other 
half,  feeding  it  exclusively  upon  earthworms. 

The  most  primitive  members  of  the  phylum 
Arthropoda  are  to  a  great  extent  cryptozoic ;  in 
Ceylon  there  is  a  species  of  the  Thysanuran 

1  Gryllotalpa  flies  to  artificial  light. 


32          EXPOSED  AND   CONCEALED  ANIMALS 

genus  Machilis  which  is  found  living  on  the 
moist  and  shaded  surfaces  of  lichen-covered  tree- 
trunks  and  rocks,  the  mottled  appearance  of 
which  the  insects  closely  resemble.  It  is  doubt- 
less in  virtue  of  this  singular  property  of  con- 
cealment that  so  many  of  the  primitive  forms 
have  survived  to  the  present  day  to  be  at  once 
the  delight  and  bewilderment  of  the  systematist. 
The  noxious  arthropods,  which  are  only  too 
familiar  in  certain  quarters,  are  also  cryptozoic, 
but  it  may  be  satisfactory  to  note  that  they  are 
not  primitive ;  they  belong  irreclaimably  to  the 
things  of  darkness. 

Earthworms  are  well  known  to  every  horti- 
culturist and  tiller  of  the  soil,  and  to  every  one 
who  has  read  Darwin's  famous  book  about  them, 
but  the  other  cryptozoa  (sensu  stricto)  are  hardly 
known  outside  the  ranks  of  specialists.  Darwin 
estimated  that  in  many  parts  of  England  more 
than  ten  tons  of  dry  earth  annually  passes  through 
the  bodies  of  earthworms  on  each  acre  of  land. 
Equally  impressive  figures  could  be  given  to 
show  the  amount  of  sand-casting  performed  by 
the  lugworms  (Arenicolidce)  on  some  temperate 
shores,  and  by  the  Enteropneusta  (Balano- 
glossus)  on  some  tropical  shores. 

So  far  I  have  referred  almost  exclusively  to 
land  animals  because  they  are  most  in  evidence 
by  the  effects  which  they  produce  in  relation  to 
husbandry.  Predatory  aquatic  animals,  especially 


CRYPTOTAXIS  33 

the  marine  mammals,  sea  snakes,  the  larger 
fishes,  free-swimming  molluscs  and  Crustacea 
(e.g.,  Portunidse,  the  swimming  crabs),  may  be 
described  as  phanerozoic  ;  but  we  may  confidently 
assert  that  in  all  cases  a  definite  cryptotactic  bias 
of  varying  intensity  could  be  demonstrated.  The 
large  Cephalopod  molluscs,  including  the  squids, 
cuttlefishes,  and  octopus,  actually  have  a  crypto- 
tactic mechanism,  the  ink-sac,  by  the  compression 
of  which  a  black  fluid  is  discharged,  effectually 
covering  the  retreat  of  the  individuals. 

Nautilus,  with  its  great  external  chambered 
shell,  has  no  ink-sac,  but  lives  in  deep  water 
and  presents  a  scheme  of  pigmentation  which 
secures  its  partial  invisibility ;  the  upper  part 
of  the  body  has  the  form  of  a  fleshy  hood  from 
the  dark-brown  surface  of  which  arise  whitish, 
wart-like  prominences  giving  a  mottled  colora- 
tion to  the  exposed  part  of  the  body,  which 
harmonises  well  with  the  zebra-like  markings  on 
the  shell.  These  simulate  the  play  of  light  upon 
the  surface  ripples  of  the  sea,  whence  I  called 
them  "ripple- markings."  On  one  occasion  I 
accidentally  dropped  a  healthy  Nautilus  overboard 
in  four  or  five  fathoms  of  very  clear  water  in 
Sandal  Bay,  Lifu  (Loyalty  Islands),  and,  not- 
withstanding the  translucency  of  the  water,  it 
disappeared  instantaneously  from  view,  and  baffled 
all  the  efforts  of  an  expert  native  diver,  who  was 

with  me  on  the  raft,  to  recover  it,  the  alternate 

C 


34         EXPOSED  AND   CONCEALED  ANIMALS 

light  and  dark  bands  on  the  shell  constituting 
a  most  effective  cryptic  device. 

The  multitudes  of  floating  organisms  which 
compose  what  is  known  as  the  plankton,  the 
primary  food  -  supply  of  sea,  river,  and  lake, 
are  often  rendered  invisible  by  their  extreme 
transparency.  The  animals  which  feed  upon 
them  do  not  pursue  them  by  the  aid  of  sight, 
but  simply  engulf  them.  Conversely,  many  of 
them  have  what  may  be  called  a  phanerotactic 
mechanism,  rendering  them  phosphorescent  and 
visible  at  night.  The  utility  of  this  mechanism 
to  the  owners  of  it  is  not  very  clear ;  it  may 
be  a  product  of  metabolism,  neither  useful  nor 
dangerous. 

The  basic  quality  underlying  all  animal  life 
is  the  cryptic,  the  fear  of  the  sun.  Basking  in 
the  sun  is  a  dangerous  pastime. 


CHAPTER   IV 

FREE  AND  FIXED  ANIMALS 
(ELEUTHEROZOA  AND  STATOZOA) 

WE  may  now  consider  briefly  another  couple 
of  essential  properties  of  plants  and  animals,  and 
note  once  more  how  the  contrasted  qualities  are 
paralleled  within  the  limits  of  the  animal  kingdom 
alone.  The  power  of  automatic  locomotion,  of 
executing  free  movements  of  translation  from 
place  to  place,  is  one  of  the  distinctive  privileges 
of  animals.  The  property  of  becoming  rooted 
to  the  soil,  and  of  undergoing  perennial  growth 
and  regeneration,  is  the  no  less  picturesque 
attribute  of  plants. 

So  inseparable  from  animal  life  did  mobility 
appear,  that  the  sponges,  hydroids,  corals,  and 
bryozoa  were  formerly  classed  together  as 
zoophytes,  and  were  either  believed  to  be  plants 
of  a  peculiar  kind  or,  as  the  name  implies,  to 
partake  of  the  nature  of  both  plants  and  animals. 
The  purely  animal  nature  of  the  coral  polyps 
was  established  in  the  middle  of  the  eighteenth 
century,  that  of  sponges  towards  the  middle  of 

35 


36  FREE  AND   FIXED   ANIMALS 

the  nineteenth.  Sponges  were  distinguished  from 
the  other  zoophytes  by  the  absence  of  polyps,  but 
at  an  early  stage  of  modern  investigation  the 
discovery  was  made  that  currents  of  water  enter 
the  body  of  the  sponge  through  small  pores  in 
its  outer  wall,  and  leave  it  through  larger  apertures 
called  oscula ;  subsequently  the  free-swimming 
ciliated  larva  was  made  known.  In  the  same 
way  the  structure  and  life-history  of  other  fixed 
or  sessile  animals  were  described,  and  it  became 
an  axiom  that  all  such  animals  have  been  derived 
from  free-living  ancestors. 

True  sessile  animals  have  been  defined  as 
those  which,  during  the  greater  portion  of  their 
lives,  are  unable  to  transport  themselves  from 
the  spot  to  which  they  have  become  attached, 
but,  unlike  parasites,  are  able  to  secure  their 
own  sustenance.  Each  phylum,  with  the  excep- 
tion of  the  craniate  Vertebrata,  has  sedentary 
members ;  all  sponges,  most  actinians  (corals  and 
sea  anemones),  and  hydroids  are  sessile ;  com- 
paratively few  echinoderms  are  fixed  at  the 
present  age,  but  "the  combined  evidence  of 
comparative  anatomy,  embryology,  and  palaeon- 
tology indicates  that  the  Echinoderma  owe  most 
of  their  obvious  characters  ...  to  their  having 
passed  through  a  pelmatozoic  (stalked)  stage, 
i.e.,  a  stage  in  which  the  animal  was  attached 
by  a  part  of  its  body  wall."1  The  recent  and 

1  F.   A.   Bather,    "Echinoderma    in    Lankester's    Treatise   on 
Zoology,"  part  iii.,  1900,  p.  3. 


SEDENTARY   HABITS  37 

extinct  echinoderms  are  accordingly  divided  into 
two  grades :  A.  Pelmatozoa ;  B.  Eleutherozoa 
(s.  str.). 

The  remarkable  prevalence  of  the  sedentary 
habit  amongst  the  lower  (i.e.  invertebrate)  animals 
seems  to  indicate  that  something  peculiarly  prim- 
ordial lies  at  the  back  of  the  phenomenon,  and 
makes  it  desirable  that  it  should  be  expressed  in 
physiological  classification.  I  venture  to  assume 
the  liberty  to  employ  two  contrasting  terms  in  a 
wide  physiological  sense,  which  have  been  used 
already  in  a  narrow  morphological  sense,  namely, 
Eleutherozoa  or  free  animals,  and  Statozoa  or 
sedentary  animals.1 

The  effects  of  the  sedentary  habit  upon 
the  growth  and  organisation  of  the  animals 
accustomed  to  it  have  been  amply  discussed  by 
Semper,  Lang,  and  others,  and  need  not  be 
described  here ;  but  it  would  be  instructive  if 
we  could  establish  the  existence  of  a  primary 
function  which  formed  the  physiological  basis 
of  all  this  varied  fixation  occurring  in  widely 
separated  morphological  groups.  There  is  an 
extra  value  in  this  inasmuch  as  we  might  by 
its  means  obtain  some  light  upon  a  number  of 
perplexing  problems  to  which  no  direct  answer 
is  possible,  e.g.,  as  to  whether  the  fixed  hydroid- 
phase  is  more  primitive  than  the  free  medusoid- 
phase  in  Hydromedusse ;  or  again  as  to  whether 

1  In  morphological  nomenclature  Statozoa  is  synonymous  with 
Pelmatozoa  (vide  Bather,  op.  tit.) 

C   2 


38  FREE   AND   FIXED   ANIMALS 

the  free  Appendiculariae  (tailed,  pelagic  Ascidians 
or  Tunicata)  are  to  be  derived  from  fixed  or  from 
free-swimming  ancestors.  These  questions  may 
be  resolved  into  one,  that  of  the  statozoic  origin 
of  Medusae  and  Appendiculariae  respectively. 
Analogy  affords  no  safe  guide  in  solving  such 
a  riddle  as  this.  There  must  be  some  common 
principle  lying  at  the  root  of  such  a  widely 
spread  phenomenon  as  that  of  the  fixation  of 
animals  ;  and  in  my  judgment  the  principle  is 
expressed  in  the  term  stereotropism  invented 
by  J.  Loeb.1 

As  a  typical  example  of  the  positive  working 
of  the  stereotropic  reaction  Loeb  quotes  Dewitz's 
account  (1885-1886)  of  the  fertilisation  of  the 
egg  of  the  cockroach.  The  spermatozoa  are 
attracted  by  any  surface  ;  when  examined  in  a 
drop  of  salt  solution  under  the  microscope  they 
are  seen  to  be  adhering  to  the  slide  and  cover- 
glass,  none  being  free  in  the  middle  of  the  drop. 
If  a  glass  bead  is  introduced  into  the  drop  they 
adhere  to  that,  never  leaving  it.  "When  once 
on  the  surface  of  the  egg,  the  spermatozoa  can 
no  more  leave  it,  but  must  move  on  its  surface 
incessantly.  In  this  way  one  spermatozoon  finally 
reaches  the  micropyle  and  gets  into  the  egg. 
The  impregnation  of  the  egg  is  therefore  in 

1  Jacques  Loeb,  "Dynamics  of  Living  Matter,"  New  York,  1906, 
p.  156.  Stereotropism  implies  contact  requirements  or  the  reaction 
to  hard  surfaces. 


STEREOTROPISM  39 

this  case  a  function  of  the  stereotropism  of  the 
spermatozoa."  [Loeb.,  loc.  cit.~\. 

As  an  instance  of  the  negative  working  of 
the  same  reaction,  Loeb  quotes  the  case  of  the 
nauplii  of  Balanus.  This  does  not  appear  to 
be  so  successful ;  it  means  either  very  little,  or 
something  which  I  do  not  understand.  Referring 
to  positive  stereotropism,  which  I  regard  as  a 
factor  of  great  potentiality,  Loeb  explains  that  it 
is  no  real  tropism  inasmuch  as  lines  of  force  do 
not  exist.  Much  less  then  must  negative  stereo- 
tropism be  responsive  to  force  of  any  descrip- 
tion ;  not  only  is  it  a  negative  quality,  it  is,  as 
we  shall  see,  practically  nothing  beyond  words. 

The  acorn  barnacles  (Balanidae)  make  up  the 
familiar  crowded  communities  of  white  shelly 
bodies  which  are  firmly  attached  to  rocks 
between  tide  marks,  and  to  other  suitable  sur- 
faces. All  barnacles  are  fixed,  but  these  are 
doubly  statozoic,  cemented  by  their  flat  base 
without  a  stalk,  in  contrast  with  the  goose 
barnacles  (Lepadidse,  pedunculate  Cirripedes) 
which  hang  by  a  stalk  attached  to  logs  and 
other  floating  objects,  sometimes  to  fishes,  sea 
snakes,  sea  birds,  and  Crustacea. 

The  stalked  barnacles,  although  fixed,  chiefly 
affect  what  may  be  termed  a  planozoic  (vagrant) 
habit  analogous,  up  to  a  certain  point,  with 
that  of  ectoparasites.  Commonly  associated 
with  floating  communities  of  Lepas,  are  two 


40  FREE   AND   FIXED   ANIMALS 

planozoic  Annelid  worms,  both  belonging  to 
the  family  Amphinomidae,  Hipponoe  gaudichaudi 
and  Amphinome  rostrata.  The  former  is  some- 
times found  between  the  valves  of  Lepas  and 
is  coloured  pinkish  orange  like  the  egg  ribbons 
of  the  barnacle.  This  association  of  forms  has 
been  met  with  on  the  Atlantic  coast  of  North 
America,1  and  I  have  obtained  them  from  the 
coast  of  Ceylon.  Other  planozoic  forms  with 
similar  distribution  associated  with  Sargassum 
or  Gulf  weed  are  the  prawn  Leander  natator 
Bate,  and  the  mollusc,  Scyllaa  pelagica,  both 
of  which  simulate  the  colour  and  foliations  of 
the  brown  seaweed  upon  which  they  live. 

Lepas  is  specially  interesting,  as  it  combines 
the  statozoic  with  the  planozoic  habit.  As 
mentioned  above,  statozoic  adults  usually  have 
free-swimming  or  pelagic,  i.e.,  pleotropic  larvae, 
and  the  nauplius  larva  of  barnacles,  at  least  in 
its  last  phase  when  it  is  called  the  metanauplius, 
already  possesses  the  primordium  of  the  static 
mechanism,  namely,  the  cement  organs  in  the 
antennules.2  Therefore  unless  there  is  some 
special  manifestation  which  Professor  Loeb  does 
not  explain,  it  would  seem  desirable  to  describe 
the  nauplius  larvae  as  positively  pleotropic,  or 

1  J.  P.  Moore,  "Some  Pelagic  Polychseta  new  to  the  Wood's 
Hole  Fauna,"  P.  Ac.  Philad.^  vol.  Iv.  part  iii.,  1903  (1904),  pp. 
793-8oi. 

2  Cf.  "  Korschelt  und  Heider.  Lehrb.  d.  vergl.  Entwicklungsge- 
schichte  der  wirbellose  Thiere,"  Jena,  1890,  p.  405. 


PLEOTROPISM  41 

simply  pelagic,  rather  than  as  negatively  stereo- 
tropic.  On  the  contrary,  it  is  the  intrinsic  and 
latent  stereotropism  of  the  larvse  which  leads 
to  the  statozoic  habit  of  the  adult. 

Furthermore  the  nauplii  of  Cirripedes  are  well 
known  to  be  markedly  heliotropic,1  and  this 
character  might  temporarily  mask  their  potential 
stereotropism,  leading  them  out  into  the  open 
sea.  Groom  obtained  them  in  large  numbers 
in  the  tow-net  at  Plymouth  during  February 
and  March,  1893,  belonging  to  a  species  which 
lives  on  the  limestone  rocks  below  the  labora- 
tory, in  places  considerably  above  high-tide 
mark,  where  they  can  only  be  moistened  by 
the  passing  foam ;  but  he  was  unable  to  rear 
them  in  confinement. 

The  Gastropod  mollusca  (snails,  slugs,  lim- 
pets), with  their  creeping  adhesive  foot,  are 
pre-eminently  stereotropic ;  yet  it  is  not  doubted 
that  the  typically  pelagic  or  pleotropic  Hetero- 
poda  and  Pteropoda  have  been  derived  from 
them  along  different  lines  of  descent.  The 
free-swimming  veliger  larva  of  mollusca  might 
be  described  equally  well  with  the  nauplius  of 
Balanus,  as  showing  negative  stereotropism,  and 
as  indicating  a  free-swimming  ancestry.  It 
seems  more  just  to  suppose  however  that  larvae 

1  i.  T.  T.  Groom.  "  On  the  Early  Development  of  Cirripedes." 
Phil.  Trans.,  London,  vol.  clxxxv.,  1894,  pp.  119-232.  2.  A.  Gruvel, 
"  Monographic  des  Cirrhipedes,"  Paris,  1905,  p.  445. 


4«  FREE  AND   FIXED   ANIMALS 

in  general  can  be  regarded  with  greater  propriety 
as  the  vehicles  of  future  adult  types  than  as 
recapitulations1  of  past  ancestors.  The  larva, 
looked  at  synthetically  in  its  entirety  as  an 
independent  organism,  apart  from  structural 
details,  is  essentially  the  vehicle  of  the  adult, 
not  that  of  an  ancestral  form.  Thus  the  frog 
doubtless  had  a  fish-like  ancestor,  but  the 
tadpole,  which  is  the  fish-like  larva  of  the  frog, 
does  not  recapitulate  the  fish-like  ancestor  of 
the  frog.  The  external  facies  of  the  tadpole  is 
common  to  the  larval  or  postlarval  stages  of 
many  fishes,  and  represents  a  fundamental  form 
likely  to  recur  by  convergence. 

The  oft-quoted  Pilidium  larva  of  Nemertine 
worms  is  an  actual  vehicle  within  human  ex- 
perience, a  vehicle  from  which  the  worm  alights 
when  it  has  reached  a  certain  stage  on  its 
journey  through  life.  The  Tornaria  larva  of 
Enteropneusta  and  the  larval  forms  of  Echino- 
derms  which  are  related  to  it  are  also  obviously 
nothing  else  but  larval,  i.e.,  not  ancestral  forms 
in  themselves.  We  may  therefore  repeat  our 
dictum  that  the  nature  of  larval  forms  is  that 
of  vehicles  of  the  future  rather  than  relics  of  the 
past,  true  larval  characters  never  having  been 
adult  characters. 

1  For  a  recent  discussion  of  the  theory  of  recapitulation  see  the 
following:  Adam  Sedgvvick,  "The  Influence  of  Darwin  on  the 
Study  of  Animal  Embryology"  in  "Darwin  and  Modern  Science," 
Cambridge,  1909. 


SWIMMING   ANNELIDS  43 

It  is  sometimes  customary  to  speak  simply  of 
positive  and  negative  reactions,  but  it  should  not 
be  forgotten  that  there  are  all  degrees  of  stereo- 
tropism,  from  an  occasional  or  facultative  mani- 
festation to  a  chronic  condition,  leading  on  to  a 
definite  sedentary  habit.  Many  cases  amongst 
the  Annelid  worms  could  be  instanced  where 
stereotropism  and  pleotropism  (the  free  swimming 
habit)  exist  side  by  side,  the  latter  frequently 
only  manifesting  itself  at  the  breeding  or  swarm- 
ing season,  as  with  epigamous  Nereids  and  the 
celebrated  Palolo  worm  (Eunicidse)  of  Samoa 
and  elsewhere  in  the  South  Seas. 

Professor  Hugo  Eisig l  has  made  exact  observa- 
tions upon  a  number  of  Annelida  Polychseta,  re- 
presenting ten  families,  some  of  which  exhibit 
a  high  degree  of  stereotropism  together  with  an 
equally  high  degree  of  pleotropism,  e.g.,  Lepid- 
asthenia  and  Ophiodromus.  Others  again  like 
Psammofyce  and  Diopatra,  never  advance  beyond 
what  is  described  as  a  swimming  gait,  i.e.,  crawl- 
ing along  the  bottom  with  a  horizontal  undulating 
movement  of  the  body  producing  alternate  arcs 
or  waves  of  progression.  When  the  amplitude  of 
these  undulations  surpasses  a  certain  magnitude, 
the  rapidity  of  movement  is  thereby  increased  to 
such  an  extent  that  the  animal  rises  from  the 
bottom  and  swims  through  the  water.  It  is  a 

1  H.  Eisig,  Ichthyotomus sanguinarius.  Monograph  28  in  "Fauna 
und  Flora  des  Golfes  von  Neapel,"  Berlin,  1906,  see  pp.  190-267. 


44  FREE   AND   FIXED   ANIMALS 

remarkable  and  suggestive  fact  that  the  typically 
pelagic  Alciopidae,  freed  entirely  from  their  stereo- 
tropic  bonds,  are  usually  very  poor  swimmers. 

The  case  of  Nephthys  is  worthy  of  special 
mention.  This  is  one  of  the  best  swimmers 
known  amongst  Annelids,  although  when  left 
alone  it  passes  most  of  its  existence  buried  in 
the  sand.  Eisig  found  that,  unlike  Psammolyce 
which  paddles  along  the  bottom  but  never 
swims,  Nephthys  on  the  contrary  undulates 
rapidly  through  the  water,  but  never  creeps  along 
the  bottom ;  it  is  either  at  rest  or  swimming, 
passing  abruptly  from  the  one  condition  to  the 
other  without  an  intervening  reptant  phase. 

An  analogous  instance  of  spasmodic  pleo- 
tropism  on  the  part  of  a  normally  stereotropic 
and  partly  statozoic  organism  is  the  case  of 
the  bivalve  mollusc,  Pecten  the  scallop.  "If 
disturbed  the  attached  scallop  can  break  or 
cast  off  its  byssal  threads  and  swim  by  clapping 
the  shell " ;  the  gaping  ventral  lips  of  the  shell 
being  directed  forwards  in  locomotion.1 

The  fact,  elucidated  by  Dr  Eisig,  that  the 
posture  of  the  Annelid  appendages  or  podia  on 
opposite  sides  of  the  arcs  is  the  same  when  the 
worm  is  progressing  slowly  along  the  bottom 
as  when  swimming  rapidly  through  the  water, 
has  led  him  to  the  conclusion  that  the  stereo- 
tropic  movement  (on  the  bottom)  is  secondary 

1  Cf.  W.  J.  Dakin,  "Memoir  on   Pecten,3'  L.M.B.C.   Memoir 
XVII.,  Liverpool,  1909. 


LEECHES  45 

as  compared  with  the  pleotropic  movement 
(through  the  water),  and  is  derived  by  inheri- 
tance from  swimmers. 

In  the  same  strain,  referring  to  Amphioxus, 
Dr  Eisig  (loc.  cit.,  p.  276)  says  that  however 
primitive  one  may  hold  its  organisation  to  be, 
yet  nobody  will  assert  that  its  limbless  body  and 
its  predominantly  cryptoid  locomotion  represent 
archaic  features.  I  am  unable  to  follow  these 
conclusions  myself,  as  I  approach  this  matter 
independently  from  a  cryptozoic  and  stereo- 
tropic  standpoint ;  and  in  this,  as  in  many 
another  affair,  everything  depends  upon  the 
point  of  view.  The  facts  are  undeniable,  but 
the  way  of  dealing  with  them  cannot  be  other 
than  arbitrary. 

Another  instructive  example  of  the  association 
of  stereotropism  of  a  highly  specialised  type  with 
well  -  marked  pleotropism  is  afforded  by  some 
leeches.  The  stereotropism  of  the  medicinal 
leech  is  of  such  a  nature  that  it  has  passed  into 
a  proverb.  Closely  connected  with  this  habit 
is  its  method  of  obtaining  its  food  by  sucking 
the  blood  of  vertebrates.  It  progresses  along  a 
hard  surface  by  the  looping  gait,  and  can  also 
swim  rapidly  by  vertical  undulations  of  the  body. 
The  land  leeches  of  Ceylon  and  Japan  belong  to 
genus  Hczmadipsa.  They  also,  as  mentioned 
above,  feed  upon  the  blood  of  vertebrates,  for 
which  they  exhibit  a  remarkable  propensity 


46  FREE   AND   FIXED   ANIMALS 

which  might  almost  be  raised  to  the  rank  of 
a  tropism.  Professor  C.  O.  Whitman  has 
published  observations  on  the  Japanese  land 
leech,1  some  of  which  I  have  confirmed  for  the 
Ceylon  land  -  leech,  including  the  fact  that  it 
cannot  swim ;  it  is  entirely  stereotropic.  It 
has  the  power  of  repairing  or  closing  severe 
wounds,  but  cannot  regenerate  the  head  or  the 
posterior  sucker.  The  lack  of  the  capacity  for 
regeneration  in  leeches  is  paralleled  by  the 
similar  behaviour  of  Amphioxus.  Leeches  offer 
as  strong  a  contrast  to  other  Annelids  in  this 
respect  as  Amphioxus  does  to  the  Entero- 
pneusta. 

The  loss  of  the  posterior  sucker  puts  an  end 
to  the  looping  gait  but  not  to  the  effort  to 
achieve  it,  and  the  lack  of  adhesion  behind  is 
compensated  by  peristalsis  on  a  level  surface, 
and  by  the  great  power  of  extension  possessed 
by  the  forebody  when  climbing  up  a  vertical 
surface.  On  placing  two  leeches  on  a  stone  in 
the  middle  of  a  large  dish  of  water  they  remained 
motionless  in  a  sub  -  erect  attitude,  resting  upon 
the  hind-sucker  for  some  time,  and  then  descended 
into  the  water,  looping  along  the  bottom  to  the 
side  of  the  vessel.  When  watching  them  by 
candle-light  they  would  not  venture  across  until 
the  light  was  removed. 

1  Quoted   in   the   Cambridge  Natural  History -,  vol.    ii.,    1896, 
"Earthworms  and  Leeches,"  by  F.  E.  Beddard,  p.  408. 


CEYLON   LAND   LEECH  47 

Whilst  sucking,  abundant  watery  fluid  issues 
from  the  epidermal  glands  of  the  Ceylon  land 
leech,  so  copious  that  it  may  flow  off  the  arm, 
upon  which  it  happens  to  be  feeding,  like  water. 
The  glands  occur  on  epidermal  papillae  which 
are  retractile,  and  can  be  protruded  simultaneously 
like  a  flush  passing  over  the  body.  No  spilling 
of  blood  takes  place  normally  ;  but  if  some  be 
taken  and  mounted  in  a  drop  of  the  epidermal 
fluid,  the  red  corpuscles  become  aggregated 
together  to  form  the  characteristic  rouleaux, 
whilst  retaining  their  shape  and  healthy  appear- 
ance. The  fluid  is  perfectly  clear,  like  the 
aqueous  humour  of  the  vertebrate  eye,  and  forms 
an  excellent  medium  for  the  examination  of  fresh 
blood. 

In  the  Japanese  land  leech  a  similar  fluid  is 
stated  to  issue  from  the  nephridial  pores.  I  have 
not  seen  this  in  Ceylon  ;  but  when  a  feeding  leech 
is  turgid  with  blood,  the  nephridiopores  become 
clearly  visible,  fourteen  pairs  situated  in  the  lower 
half  of  the  lateral  pale  bands  at  the  posterior 
borders  of  the  segments,  the  space  of  five  annuli 
intervening  between  successive  pores.  There 
are  three  segments  or  fifteen  annuli  in  front  of 
the  first  pore ;  the  posterior,  nuchal  eyes  appear 
to  lie  on  the  second  annulus.  The  erectile 
papillae  are  arranged  in  a  single  row  across  each 
annulus.  Certain  of  these  papillae  have  a  special 
character  and  occur  in  longitudinal  rows,  lying 


48  FREE   AND   FIXED   ANIMALS 

singly  in  the  first  annulus  of  each  segment  (i.e., 
in  the  annulus  following  that  which  carries  the 
segmental  nephridiopores) ;  each  shows  a  dark 
centre  surrounded  by  a  pale  border  on  the  top 
of  a  retractile  papilla.  These  segmental  ocelli- 
form  organs  commence  in  the  segment  in  front 
of  the  first  nephridiopore,  and  there  are  twelve 
of  them  in  each  segment,  i.e.,  six  on  each  side 
disposed  as  follows  :  two  lateral  rows,  one  above 
and  one  below  the  lateral  band,  a  dorso-lateral 
and  a  ventro  -  lateral  row,  a  submedian  dorsal 
and  a  submedian  ventral  row. 

The  preceding  notes  serve  to  illustrate,  in 
respect  of  external  characters  alone,  the  highly 
specialised  organisation  of  the  land  leech,  which 
is  already  indicated  by  the  concentration  of  its 
stereotropic  mechanism,  by  its  lack  of  regenera- 
tion, and  by  its  passion  for  blood  which  amounts 
to  a  hsematotropism,  as  no  one  can  doubt  who 
has  read  Sir  Emerson  Tennent's  account  of  it, 
or  who  has  experienced  it  in  the  flesh.  Hardly 
anything  proclaims  a  finished  organisation,  the 
culmination  of  a  phyletic  career,  so  plainly  as 
an  exclusive  diet.  In  point  of  fact  the  leeches 
represent  the  culmination  at  the  present  age  of 
the  Annelid  branch  in  a  definite  direction. 
Branches  of  the  tree  of  life  often  terminate  in 
a  brush  of  points,  not  necessarily  in  a  single 
point ;  and  leeches  constitute  one  of  these 
terminations, 


TREE   SNAKES  49 

Few  free-living  animals  except  terrestrial 
snakes,  are  so  exquisitely  stereotropic  as  are 
leeches ;  and  snakes  offer  a  further  analogy 
with  Annelids  as  a  whole  in  the  combination 
of  stereotropism  and  pleotropism  in  some  fresh- 
water species,  stereotropism  alone  in  burrowing 
and  in  arboreal  species,  pleotropism  alone  in 
many  marine  species.  In  some  tree  snakes,  e.g., 
Dendrophis,  Dendrelaphis,  and  Chrysopelea,  there 
is  a  lateral  keel  or  suture  on  each  side  of  the 
ventral  shields.  It  has  been  suggested  by  Mr 
R.  Shelford1  that  this  carination  or  hinging  01 
the  ventral  shields,  by  allowing  the  body  to 
flatten  itself  out  and  become  concave  below, 
enables  the  snake,  when  springing  from  a  height, 
to  descend  gently  like  a  parachute.  Against  this 
it  has  to  be  noted  that  burrowing  snakes  like 
Cylindrophis,  which  have  very  small  ventral 
shields,  are  able  to  flatten  the  body  in  a 
marvellous  manner.  I  rather  think  that  the 
lateral  hinge-lines  are  in  the  first  instance  an 
accessory  stereotropic  contrivance  enabling  the 
snakes  to  climb  up  rough  tree-trunks  with  greater 
facility;  moreover,  a  lateral  angulation  of  the 
ventral  shields  is  found  in  the  terrestrial  genera, 
Lycodon  and  Hydrophobus.  Nevertheless,  the 
possibility  is  not  excluded  that  they  might  also 
serve  on  occasion  for  the  parachute  flight ;  but 

1  R.   Shelford.     A  note  on  "Flying"   Snakes,  P.  Zool.   Soc.> 
London,  1906,  i.,  pp.  227-230. 

P 


50  FREE   AND   FIXED   ANIMALS 

that  the  latter  ever  really  takes  place  is  a  matter 
which  requires  confirmation. 

Other  reptiles  which  take  refuge  in  trees,  e.g., 
Calotes  and  Varanus,  when  alarmed,  will  project 
themselves  from  a  great  height  to  the  ground 
without  elaborating  a  mechanism  for  aerial  flight. 
The  case  of  Draco,  the  flying  lizard  of  Malaya, 
where  such  a  mechanism  is  developed  (dermal 
membrane  supported  by  ribs),  belongs  to  a 
different  category ;  and  the  facts  seem  to  show 
clearly  that  it  is  not  merely  the  habit  of  taking 
flying  leaps,  like  monkeys,  for  example,  that  has 
led  to  the  formation  of  organs  of  flight. 

We  may  now  submit  the  conclusion  that  just 
as  all  divisions  of  the  animal  kingdom  display 
a  cryptozoic  bias,  so  do  they  all  show  a  statozoic 
tendency.  The  sedentary  habit  is  referable  to 
a  stereotropic  basis,  and  pelagic  or  pleotropic 
forms  belonging  to  groups  which  are  pre- 
dominantly sedentary,  have  had  a  probable 
statozoic  origin.  All  land  animals  have  had 
aquatic  ancestors,  or,  more  precisely,  all  air- 
breathers  have  descended  from  water-breathers 
as  defined  above.  The  latter  are  primarily 
aquatic,  and  are  either  stereotropic  at  the  present 
time,  or  it  may  be  argued  that  they  have  had 
a  more  or  less  remote  stereotropic  ancestry. 

Thus  the  permanent  fixation  of  so  many 
aquatic  animals  is  not  such  a  bizarre  phenomenon 
as  it  appeared  to  be  to  the  older  naturalists 


COMMON  PROPERTIES  51 

of  the  eighteenth  century,  but  it  is  merely 
an  extreme  manifestation  of  a  very  primitive 
property  of  animal  life.  My  coupling  of  the 
cryptozoic  habit  with  the  stereotropic  reaction 
is  incidentally  justified  by  a  statement  of  Eisig 
to  the  effect  that  stereotropism  signifies  rest 
and  concealment.  The  purpose  of  this  and  the 
preceding  chapter  has  been  to  insist  upon  the 
fundamental  nature  of  cryptotaxis  and  stereo- 
tropism in  the  convergent  evolution  of  animals. 
These  are  some  of  the  primary  properties  of 
living  matter  upon  which  the  moulding  forces 
of  nature  have  been  at  work  for  untold  ages. 

A  somewhat  analogous  case,  in  so  far  as  it  is 
an  extreme  manifestation  of  a  common  property, 
is  the  electric  power  of  the  three  typical  kinds 
of  electric  fishes  (ray,  eel  of  South  American 
rivers,  and  catfish  of  African  rivers).  As  shown 
by  E.  du  Bois-Reymond,1  these  fishes  owe  their 
capacity  for  imparting  severe  shocks,  in  different 
directions,  to  an  intensification  of  the  common 
electromotive  properties  of  nerve  and  muscle. 
In  each  case,  as  pointed  out  by  Dr  D.  S.  Jordan,2 
closely  -  related  species  show  no  trace  of  the 
electric  endowment. 

1  See  "Biological  Memoirs,"  Oxford,  1887,  vol.  i. 

2  "Guide  to  the  Study  of  Fishes,"  1905. 


CHAPTER  V 

MIMICRY   AND    HOMOPLASY 

THE  term  convergence  is  applied  to  resemblances 
amongst  animals  which  are  not  due  to  direct 
relationship  or  genetic  affinity ;  in  other  words, 
which  are  not  derived  by  inheritance  from 
common  ancestors,  but  which  result  from  inde- 
pendent functional  adaptation  to  similar  ends : 
e.g.,  the  exuviation  and  regeneration  of  newts 
as  compared  with  the  same  phenomena  in  crabs 
and  lobsters,  or  again  the  coiled  prehensile  tail 
and  great  rolling  eyes,  moving  independently, 
of  Hippocampus  and  Chameleons.1 

It  has  long  been  recognised  as  a  dominant 
factor  in  comparative  morphogeny.  Common 
characters  of  adaptation  in  different  animals 
may  be  due  to  common  inheritance,  or  they 
may  be  due  to  convergence.  Hence  it  follows, 
in  Semper's  words,  that  the  problem  of  the 
morphologist  is  to  learn  to  distinguish  such 

1  These  two  striking  instances  of  double  convergence  were 
presented  in  respective  juxtaposition  by  Mr  Frank  Buckland. 
("Curiosities  of  Natural  History."  Reprinted  1903,  Macmillan, 
First  and  Second  Series.)  Compare  the  case  of  the  egg-laying, 
duck-billed  Platypus  of  Australia. 

52 


WAYS   OF   CONVERGENCE  53 

characters  as  have  been  developed  by  adapta- 
tion, quite  independently  of  the  affinities  of 
animals,  from  such  as  have  been  transmitted  by 
inheritance  through  a  long  series  of  generations.1 

Morphology  therefore  naturally  falls  into  two 
divisions :  convergent  morphogeny  and  homo- 
genetic or  normal  morphogeny.  The  former  is 
not  generally  admitted  as  a  legitimate  branch 
of  positive  morphology  ;  the  latter  is  the  way 
usually  followed,  and  common  characters  which 
are  not  homologous  are  sometimes  attributed 
to  "mere  convergence."  It  appears  that  there 
is  more  joy  amongst  morphologists  over  one 
attempt  at  genealogy  than  over  ninety  and  nine 
demonstrations  of  convergence.  Personally,  I 
take  the  view  that  both  of  the  above-named 
divisions  of  morphology  are  equally  important. 

There  are  several  ways  of  convergence  and 
each  is  expressed  in  various  degrees  of  intensity. 
The  two  most  widely  diffused  ways  of  con- 
vergence are  known  respectively  as  homoplasy2 
and  mimicry. 

True  mimicry  is  defined  by  Mr  Alfred  Russel 
Wallace3  as  "a  form  of  protective  resemblance 
in  which  one  species  so  closely  resembles 
another  in  external  form  and  colouring  as  to 

1  Karl  Semper,  "  The  Natural  Conditions  of  Existence  as  they 
affect  Animal  Life."    Fourth  edition,  London,  1890. 

2  The  term  and  idea  of  homoplasy  were  introduced  by  Sir  E. 
Ray  Lankester  :  "  On  the  Use  of  the  Term  Homology  in  Modern 
Zoology,"  Ann.  Nat.  Hist.,  1870. 

3  A.  R.  Wallace,  "  Darwinism."    Second  edition,  London,  1890. 

D    2 


54  MIMICRY   AND   HOMOPLASY 

be  mistaken  for  it,  although  the  two  may  not 
be    really   allied   and   often    belong    to    distinct 
families  or  orders."     The  first  constant  condition 
under  which  true  mimicry  occurs   is   "that  the 
imitative   species   occur   in   the   same   area   and 
occupy  the  very  same  station  as  the  imitated." 
The    last    essential    condition    of    mimicry,    as 
explained  by   Mr  Wallace,   is   "that   the  imita- 
tion,   however    minute,    is   external   and   visible 
only,  never  extending  to  internal  characters  or 
to  such  as  do  not  affect  the  external  appearance." 
Furthermore,  it  is  explained  that  warning  colora- 
tion is  the  basis  of  the  phenomenon  of  mimicry, 
but   it   is    not   stated  what   lies   at   the   root   of 
warning  coloration.     Of  course  it  is  fostered  by 
natural  selection,  but  we  do  not  know  the  funda- 
mental  reaction  that   determines   it.      Professor 
Poulton1    adds    as    another    attribute    of    true 
mimicry  that    it    is   a   sham,   that    is   to    say,   a 
false  alarm  ;    true  warning  colours,  occurring  in 
unpalatable  or  dangerous  bodies,  are  genuine. 

Warning  coloration,  mimicry,  protective  re- 
semblance, and  the  death  feint,  are  facts  in 
nature  which  are  known  to  have  been  doubted 
in  the  past  by  "armchair  philosophers."  This 
has  doubtless  been  due  to  the  fact  that  some 
delicate  shade  of  meaning  has  often  been  over- 
looked in  the  original  descriptions.  Even  if 

1  E.  B.  Poulton,  "Colours  of  Animals,"  Encyc.  Brit.,  ninth 
edition,  Supplement. 


MOMENTS  OF  CONVERGENCE  55 

both  form  and  colour  of  a  mimic  harmonise 
with  its  model,  there  must  still  be  some  slight 
movement,  some  particular  action  or  habit  to 
complete  the  illusion.  Thus,  to  take  one  example 
only  as  the  type  of  a  long  series,  there  occurs 
in  Ceylon  a  longicorn  beetle,  Callichroma  chryso- 
gaster,  sufficiently  rare  to  satisfy  Wallace's  con- 
dition that  the  imitators  are  always  less  numerous 
than  the  imitated,  which  shows  characteristic 
waspish  marks,  orange  yellow  antennae  and  legs 
set  upon  a  body  uniformly  bluish  black  above ; 
but  it  only  resembles  a  wasp  when  it  is  in  flight 
and  at  the  moment  of  alighting  on  a  bush  in  full 
sunlight;  when  pinned  in  a  box  it  is  a  "mere 
beetle." 

Amongst  insects  especially  there  are  many 
examples  of  forms  belonging  to  different  orders, 
protected  by  different  noxious  secretions,  and 
yet  possessing  common  warning  colours,  which 
do  not  occupy  identical  stations.  Thus  a  Reduviid 
bug  living  under  dry  bark  may  resemble  a  Bom- 
bardier beetle  living  under  damp  logs.  This 
belongs  to  Poulton's  category  of  synaposematic 
resemblances  "such  as  obtain  between  animals 
of  all  kinds  adopting  sematic  [signalling]  methods 
of  defence  in  the  same  country " ;  this  is  some- 
times known  as  Miillerian  mimicry,  as  dis- 
tinguished from  true  or  Batesian  mimicry.  The 
common  cryptic  or  protective  coloration  of  many 
animals  (oceanic,  deserticolous,  lichenicolous)  in 


56  MIMICRY   AND   HOMOPLASY 

a  special  environment  produces  likenesses  among 
different  animals  which  are  incidental  and  ana- 
logous to  those  convergent  resemblances  "caused 
by  functional  adaptation,  such  as  the  mole-like 
forms  produced  in  the  burrowing  Insectivora, 
Rodentia,  and  Marsupialia"  [Poulton]. 

We  may  safely  claim  that  the  possession  by 
noxious  animals  of  common  warning  coloration  is 
as  much  due  to  convergence  as  is  the  possession 
by  harmless  animals  of  a  common  protective 
coloration  ;  and  both  of  these  colour  -  schemes 
are  referable  to  conceivable  though  indefinite 
reactions.  On  the  other  hand,  the  resemblances 
and  associations  between  palatable  and  unpalat- 
able insects  are  hard  to  explain  on  the  tropism 
theory,  unless  we  suppose  that  they  arose  by 
ordinary  convergence  before  advantage  was  taken 
of  them  by  natural  selection.  We  may  there- 
fore differentiate  usefully  between  passive  or  con- 
vergent mimicry  (including  Poulton's  syncryptic 
and  synaposematic  categories)  and  active  or 
selective  mimicry  (including  true  mimetic  re- 
semblances) ;  and  we  may  add  that  while  in 
typical  cases  the  categories  are  sharply  divided, 
in  less  obvious  cases  it  is  often  as  difficult  to 
separate  pure  convergence  from  either  form  of 
mimicry  as  it  is  to  distinguish  it  from  homology  ; 
the  temptation  being  to  describe  the  more  or 
less  sensational  instances  and  to  ignore  those 
which  are  less  convincing,  thereby  imparting  a 


PHYSIOGNOMY  57 

one-sided  and  exaggerated  impression  to  a  very 
common  natural  phenomenon. 

Mimicry,  in  the  wide  sense  of  the  term,  in- 
volves the  entire  superficial  aspect  of  the  body ; 
the  moment  the  details  are  analysed  the  like- 
ness disappears ;  it  is  absolutely  essential  to  its 
success  that  structural  details  should  be  disre- 
garded. We  may  define  mimicry  broadly  as  a 
physiognomical  convergence  between  two  or 
more  species  of  animals,  in  this  way  distinguish- 
ing it  from  homoplasy,  which  depends  upon  a 
more  deep  -  seated  organic  or  structural  con- 
vergence. Such  coincidences  of  form  and 
function  as  those  presented  by  the  mole  -  like 
burrowing  mammals  referred  to  above,  by  other 
marsupial  and  placental  mammals,  and  by  some 
arboreal  mammals,  e.g.,  Tupaiidse  (tree-shrews) 
and  Sciuridae  (squirrels),  the  flying  Insectivore 
Galeopithecus  and  the  flying  squirrel  Pteromys, 
in  so  far  as  the  whole  bodily  appearance  is 
involved,  afford  examples  of  a  third  way  of 
convergence  known  as  parallel  evolution ;  it  will 
soon  be  clear  how  convergence  is  not  incom- 
patible with  parallelism.  They  resemble  each 
other  because  of  their  approximation  to  funda- 
mental forms,  e.g.,  the  burrowing  form,  the 
arboreal  form,  and  so  on.  A  carnivorous  Mar- 
supial must  necessarily  bear  some  resemblance 
to  a  true  Carnivore,  the  fact  being  that  it  is* 
an  extraordinarily  close  one. 


58  MIMICRY  AND   HOMOPLASY 

There  are  very  many  examples,  chiefly  amongst 
insects  and  other  arthropods,  at  all  stages  of 
growth  and  metamorphosis,  of  resemblances  to 
fragments  of  vegetation,  excrescences  on  bark, 
droppings,  etc.  It  is  usually  not  difficult  to  dis- 
criminate between  effective  resemblances  which 
are  objective,  and  fanciful  resemblances  which 
are  subjective.  With  the  latter  I  have  nothing 
to  do  here,  although  they  are  sometimes  of  such 
a  nature  as  to  have  given  rise  to  enduring  popular 
traditions,  and  on  that  account  are  entitled  to 
respect  within  their  own  scope. 

The  resemblance  to  a  common  fundamental 
form  is  well  illustrated  by  the  leaf-mimics,  of 
which  the  best  known  are  the  Leaf  Butterflies 
and  the  Leaf  Insects.  Mr  Wallace  remarks 
that  "  many  butterflies,  in  all  parts  of  the  world, 
resemble  dead  leaves  on  their  under  side,  but 
those  in  which  this  form  of  protection  is  carried 
to  the  greatest  perfection  are  the  species  of  the 
Eastern  genus  Kallima"  This  genus  of  butter- 
flies is  noted  for  the  extreme  amount  of  indi- 
vidual variation  in  the  markings  on  the  under 
side  of  the  wings,  simulating  all  degrees  of  decay 
and  discoloration  and  fungus-attack  ;  to  this  must 
be  added  the  perfect  leaf-shape  and  veining  of 
the  closed  wings.  Besides  all  this,  its  method 
of  alighting  is  such  as  to  complete  the  illusion. 
Referring  in  particular  to  the  Sumatran  species 
Wallace  says  that  "this  is  effected  by  the  butter- 


LEAF   BUTTERFLIES  59 

fly  always  settling  on  a  twig,  with  the  short  tail 
of  the  hind-wings  just  touching  it  and  forming 
the  leaf -stalk."  In  Sumatra  he  has  often  seen 
one  enter  a  bush  and  then  disappear  like  magic. 
In  this  account  one  may  wonder  why  the  butter- 
fly resembles  a  dead  leaf  seeing  that  it  rests  upon 
a  twig  in  the  attitude  of  a  normal  leaf;  perhaps 
it  may  be  explained  by  the  circumstance  that 
in  tropical  vegetation  a  regular  leaf  -  fall  is  the 
exception  instead  of  being  the  rule  as  it  is  in 
temperate  climates,  and  it  is  a  very  common  thing 
to  see  a  few  conspicuously  discoloured  leaves  still 
attached  to  the  branches  in  the  midst  of  green 
foliage. 

The  Kallima  philarchus  of  Ceylon  seems  to 
behave  rather  differently  since,  as  Mr  E.  E. 
Green  has  noticed  and  recorded,  it  "  more  usually 
settles  head  downwards  on  the  trunk  of  a  tree, 
.  .  .  swaying  gently  from  side  to  side.  It  might 
then  be  mistaken  very  easily  for  a  detached 
leaf  that  in  its  fall  has  hitched  up  in  a  cobweb 
and  is  being  shaken  by  the  breeze."1  Another 
observer,  Mr  W.  A.  Cave,2  reports  that  he  had 
exceptional  opportunities  for  watching  one  of 
these  butterflies  at  close  quarters,  as  it  frequented 
a  certain  spot  on  the  trunk  of  a  tree  for  two  days  ; 
as  soon  as  it  settled,  "  which  it  did  in  the  usual 
way,  it  immediately  turned  round  so  that  its 
head  pointed  downwards."  Mr  Cave  was  much 

1  E.  E.  Green,  "Mimicry  in   Insect   Life,  as  exemplified  by 
Ceylon  Insects,"  Spolia  Zeylanica,  vol.  v.,  1908,  see  p.  89. 
*  W.  A.  Cave,  Note  on  Kallima  philarchus •,  t.  <:.,  p.  142. 


60  MIMICRY   AND   HOMOPLASY 

puzzled  over  this  proceeding,  which  occurred  every 
time  the  butterfly  settled,  until  he  discovered  the 
reason:  "the  butterfly  turned  round  so  that  the 
tail  of  its  two  hind  wings  would  almost  come  into 
contact  with  the  trunk  of  the  tree,  thus  representing 
a  stalk,  and  the  apparently  dead  leaf  would  hang 
in  a  perfectly  natural  way,  drooping  downwards." 

A  Mexican  leaf  butterfly  (Tagetis  mermeria), 
according  to  Mr  C.  W.  Beebe,  behaves  in  such 
a  manner  as  to  cause  itself  to  be  mistaken 
at  first  glance,  not  only  for  a  fallen  but  for  a 
falling  leaf  as  well  Here  also  the  individual 
variation  is  very  great,  in  correspondence  with 
the  "variety  of  hues  and  mottlings  which  exist 
among  dead  and  withered  leaves.  .  .  .  When 
the  insect  took  to  wing  it  shot  almost  straight 
upward,  and  instantly  attained  the  highest  point 
of  its  flight.  From  here  to  its  place  of  alighting 
its  course  was  a  gradual  descent — this  living  leaf 
unconsciously  reflecting  every  detail  of  the  fall 
of  the  withered  bits  of  vegetation.  And  further, 
when  the  butterfly  alighted,  it  was  not  with  a 
fluttering  and  a  few  moments  of  hovering,  but 
as  a  leaf  comes  to  rest,  so  the  insect — a  sudden 
drop  to  the  very  ground,  wings  snapped  together, 
and  the  apparently  dried,  worm-eaten  leaf  leaned 
far  over  to  one  side  and  swayed  with  every 
breath  of  air."1 

It  is  interesting  to  remark  that  a  notion  exists 
to  the  effect  that  the  constant  repetition  of  such 

1  C.  William  Beebe,  "Two  Bird- Lovers  in  Mexico,"  Boston 
and  New  York,  1905 ;  see  pp.  241-243. 


LEAF   INSECTS  61 

considerable  variations  as  are  met  with  in  leaf- 
like  Lepidoptera  and  Orthoptera,  from  genera- 
tion to  generation,  is  a  standing  witness  against 
the  truth  of  "  Darwinism,"  inasmuch  as,  accord- 
ing to  the  Darwinian  theory,  such  variations 
ought  either  to  become  fixed  by  natural  selection 
or  swamped  by  interbreeding.  On  the  face  of 
it  there  would  seem  to  be  some  force  in  this 
argument.  Whatever  the  answer  may  be,  it  is 
not  that  these  varieties  may  become  fixed  in 
course  of  time  ;  on  the  other  hand,  it  may  be  that 
natural  selection  is  interested  in  keeping  alive  the 
variations  for  the  benefit  of  the  species,  not  for 
the  production  of  new  species.  In  any  case  it  is 
a  good  point,  worthy  of  special  consideration. 

Butterflies  owe  their  leaf-like  appearance  to 
the  effect  of  bilateral  compression  when  the 
wings  are  closed.  Leaf  insects  of  the  genus 
Phyllium,  which  belong  to  the  order  Orthoptera 
(containing  also  the  cockroaches,  grasshoppers, 
and  stick-insects),  owe  it  to  the  dorsiventral 
flattening  of  the  body.  A  similar  antagonistic 
flattening  of  the  body  occurs  between  two 
human  ectoparasites  where  the  compression 
also  achieves  a  cryptic  object  indirectly,  viz.  : 
the  bilaterally  flattened  Pulex  and  the  dorsiven- 
t rally  flattened  Cimex. 

An  excellent  description  and  plate  of  Phy Ilium 
crurifolium,  a  wonderful  leaf  insect  which  is 
found  in  the  Seychelles  islands  as  well  as  in 
Ceylon,  has  been  published  recently  by  Mr 


62  MIMICRY   AND   HOMOPLASY 

H.  S.  Leigh,1  whose  account,  based  on  material 
obtained    from     the     Seychelles     and    bred    in 
England,   I  can  confirm,  having  reared  them  in 
Ceylon   for    several    years.      The    body   of   the 
males  is   generally  green  with   a   pair   of  clear 
ocellations   on    the   fourth    abdominal    segment ; 
they  vary  very  little  in  colour ;  they  have  small 
tegmina   and   large   wings,    and   are   capable   of 
flight ;  their   legs    break   off  by   autotomy   with 
the   greatest    ease.     The    flightless   females,    on 
the  contrary,  appear  in  two  principal  varieties, 
green    and    red    (ferruginous),    the    green    indi- 
viduals   predominating    over    the    russet,    but    I 
have   been    unable    to    determine    any   constant 
numerical   relation    between    them.     The    russet 
forms    are    more    variegated    and    show    more 
individual  variation  than  the  green  forms.     Out 
of  one    hundred    and    eight    individuals    reared 
from    eggs    laid    by   green    females,    six    russet 
forms  appeared  after   the  later   moults ;   out   of 
seventy-five   individuals   reared   from    eggs   laid 
by  two  russet  females,  one  russet  form  appeared. 
The   female    leaf   insects    therefore    resemble 
both  green  and  sere  leaves  in  about  the  same 
proportion  as  the  latter  occur  on  green  trees  in 
the  tropics.     Not  only  this,  but  when  immature 
they  can  fold  up   their   abdomen  like  a   curled 
leaf;  and  at  all  stages  they  sometimes  sway  like 
leaves  in  a  breeze,   and  at  other  times  remain 

1  H.  S.  Leigh,  "  Preliminary  Account  of  the  Life-History  of  the 
Leaf  Insect,  Phyllium  crurifolium  Serville,"  P.  Zool.  Soc.^  London, 
1909,  pp.  103-113. 


AUTOTOMY  63 

rigid  with  fore-limbs  extended  like  leaves  in  a 
calm.  The  adult  females,  as  duly  noted  by 
Leigh,  are  more  sluggish  than  the  males ;  and 
perhaps  connected  with  this  circumstance  is 
their  much  lower  capacity  for  autotomy  of  the 
appendages.  A  female  can  be  held  by  the  legs 
without  discarding  them,  but  if  a  male  be  so 
held  mutilation  ensues.  The  eggs  of  PhylKum, 
as  of  all  Phasmidae,  resemble  plant  seeds.1 

I  may  mention  here  an  observation  bearing 
upon  the  autotomy  of  the  wings  of  termites. 
When  they  alight  upon  the  ground  after  swarm- 
ing out  of  the  nest,  it  is  well  known  that  they 
throw  off  their  wings  by  a  special  act  of  autotomy; 
but  if  seized  by  the  wings  before  the  performance 
of  this  act,  they  cannot  accomplish  it  so  long  as 
the  wings  are  held.  It  is  the  same  when  only  one 
wing  is  taken  between  the  fingers ;  the  autotomy 
is  inhibited.  The  casting  of  the  wings  by  termites 
and  by  ants  is  of  course  not  followed  by  a 
corresponding  act  of  regeneration,  such  as  follows 
upon  the  autotomy  of  the  legs  of  immature 
phasmids ;  mature  male  phasmids,  as  we  have  seen 
in  the  case  of  the  leaf  insect,  cast  their  legs  but 
will  not  live  long  enough  to  regenerate  them. 

Amongst   fishes   the   Australian    "  Sea-horse " 

1  The  newly-hatched  young  of  Phyllium  crurifolium  present  a 
warning  scheme  of  coloration  which  disappears  with  the  first 
moult,  blackish  head  and  legs  and  a  scarlet  abdomen.  I  brought 
a  quantity  of  eggs  recently  from  Ceylon  to  England,  some  of 
which  hatched  out  during  the  voyage,  others  later  in  the  Zoological 
Gardens.  On  showing  them  fresh  out  of  the  egg  to  my  friend  Mr 
R.  I.  Pocock,  he  remarked  instantly  upon  their  striking  resemblance 
to  certain  plant  bugs. 


64  MIMICRY  AND  HOMOPLASY 

Phyllopteryx,  of  which  there  is  a  figure  in  Dr 
Giinther's  "  Study  of  Fishes  "  (1880),  is  furnished 
with  cutaneous  processes  which  resemble  trailing 
seaweed.  Another  fish,  during  a  limited  period 
of  its  life-history,  namely,  about  the  yearling 
stage,  effectively  resembles  a  dead  water-logged 
leaf.  This  is  the  young  of  the  so-called  sea- 
bat,  Plat  ax  vesper  tilio?  The  mature  fish  is 
nearly  uniformly  dark-coloured ;  it  attains  a 
moderate  size  and  is  very  high  in  the  body, 
which  is  strongly  compressed.  It  swims  about 
in  small  shoals  and  has  the  habit,  possessed  by 
many  other  similarly  constituted  fishes,  of  turning 
over  on  one  side  momentarily  whilst  swimming. 
In  the  young  stage,  when  the  total  height  of 
body  and  fins  is  about  two  or  three  inches, 
the  colour  is  pale  yellowish  or  brownish  with 
variegated,  irregular  and  variable  markings. 
When  seen  living  in  the  water,  its  resemblance 
to  a  sere  leaf  is  extraordinary ;  when  taken  out 
of  water  it  loses  something  of  the  strangeness 
of  its  appearance,  and  when  preserved,  the  fins 
collapse,  thereby  destroying  the  living  aspect  of 
the  animal. 

In  February  1904  I  had  the  opportunity  of 
seeing  the  peculiar  movements  of  a  young  Platax 
under  normal  conditions,  off  the  west  coast  of 
Ceylon.  Whilst  a  fisherman  was  attempting  to 
catch  it  with  a  pole-net,  it  suddenly  toppled  over 

1  A.  Willey,  "  Note  on  Leaf-Mimicry,"  Spolia  Zeylanica,  vol.  ii., 
1904,  pp*  51-55.    See  also  Nature^  vol.  Ixxx,,  1909,  p.  247. 


LEAF   FISHES  65 

and  sank  gently  and  inertly  to  the  bottom  like 
a  yellow  leaf,  for  which  I  mistook  it  at  first.  As 
I  was  about  to  turn  away  from  such  a  common- 
place object  as  a  drenched  leaf,  it  righted  itself 
once  more  and  darted  away.  It  was  subsequently 
captured  and  sketched. 

Fallen  leaves  of  various  kinds  are  commonly 
seen  drifting  about  in  the  sea,  sometimes  at  a 
considerable  distance  offshore.  The  most  familiar 
examples  of  such  drifting  leaves  are  those  of  the 
mangrove  trees  which  fringe  the  borders  of 
estuaries  and  backwaters  in  the  tropics.  This 
singular  Leaf  Fish,  besides  having  the  requisite 
shape  and  colour,  has  also  the  power  of  assuming 
the  inertia  of  a  dead  leaf,  or  of  feigning  death 
in  the  sea,  thus  completing  the  disguise.  The 
contour  of  its  body  is  strengthened  behind  by 
a  line  of  dark  pigment,  which  passes  along  the 
hinder  margins  of  the  dorsal  and  anal  fins,  but 
through  the  base  of  the  caudal  fin  ;  the  latter  is 
unpigmented,  hyaline  and  invisible  under  water. 
The  pectoral  fins  are  also  transparent,  but  the 
elongated  ventral  fins  are  opaque,  showing  the 
yellow  ground  colour  and  a  dense  outer  border 
conterminous  with  the  border  of  the  anal  fin. 
There  can  be  no  mistake  as  to  the  effective- 
ness of  this  case  of  leaf-mimicry  under  water; 
moreover,  the  surface  of  the  body  shows  lines 
of  pigment  and  small  spots  such  as  are  seen 
in  a  decaying  leaf  (Fig.  i). 

The  three  different  types  Phyllium,  Kallima, 

E 


66 


MIMICRY   AND   HOMOPLASY 


and   Platax,    which   have    so   little    in    common 
otherwise,   agree   in   exhibiting   the   shape  of  a 


FiG.  I.     The  Leaf  Fish  or  yearling  stage  of  Platax 
vespertilio.     Original  from  Spolia  Zeylanica. 

simple  bifacial  leaf,  one  of  the  commonest 
objects,  dead  or  alive,  ashore  or  afloat,  in  nature. 
Yet  they  do  not  effectively  resemble  each  other 
except  in  an  abstract  manner  by  conforming  to 
a  fundamental  pattern,  i.e.,  by  convergence. 
The  death  feint  is  a  common  and  very  widely 


COINCIDENCE  AND  CORRELATION  67 

spread  method  of  protection  from  predatory 
enemies.  It  is  an  extreme  manifestation  of  the 
virtue  of  immobility  which  is  equally  well  known 
to  hunters  and  to  their  quarry.  When  it  is 
associated  with  singularities  of  form  and  colora- 
tion, it  produces  a  combination  which  is  so 
remarkable  that  it  is  generally  received  with  a 
proper  amount  of  scepticism.  The  resemblance 
which  these  animals  bear  to  a  leaf  is  a  selective 
one ;  that  which  they  bear  incidentally  to  each 
other  is  a  convergent  one,  due  to  their  inde- 
pendent acquirement  of  a  common  fundamental 
form.  In  these  cases  it  may  be  supposed  that 
natural  selection  has  taken  advantage  of  a 
previously  existing  aptitude,  the  essence  of  the 
leaf-form  being  present  as  a  generic  character 
before  the  actual  assumption  of  the  intimate 
resemblance  became  fixed. 

Other  striking  convergences  depending  upon 
form  and  coloration  are  those  which  obtain 
between  some  harmless  and  some  poisonous 
snakes  inhabiting  the  tropical  regions  of  both 
hemispheres.  Darwin  described  the  examples 
from  South  America.  The  mutual  resemblances 
proceed  largely  from  the  character  of  the  trans- 
verse banding  of  the  body  which  they  have  in 
common ;  and  the  colour  coincidence  may  be 
so  close  that  in  a  rapid  sorting  the  species 
would  be  thrown  together  and  confounded  one 
with  another. 

Lastly,  reference  should  be  made  to  the  well- 


68  MIMICRY  AND   HOMOPLASY 

known  cases  of  plumage  resemblances  amongst 
birds  and  to  the  longitudinal  striping  of  some 
mammals,  e.g.,  young  wild  pig,  young  tapir, 
mouse-deer,  striped  squirrels,  etc.  In  all  these 
cases  convergence  is  independent  of  selection. 

In  order  to  guard  against  conveying  a  wrong 
impression,  it  may  be  well  to  add  that  in  cases 
of  double  or  multiple  convergence  such  as  are 
mentioned  on  p.  52,  there  is  no  a  priori  pre- 
sumption of  correlation  between  the  several 
characters  which  are  repeated  in  the  different 
types.  Thus  there  is  no  correlation  between 
the  separately  moving  eyes  of  Hippocampus  and 
its  prehensile  tail,  because  other  Lophobranchiate 
fishes  (Pipe-fishes),  which  have  a  straight  tail,  roll 
their  eyes  in  the  same  way,  as  can  be  seen  very 
prettily  in  the  Naples  Aquarium.  The  property 
which  these  fishes  present  in  common  with 
chameleons  is  the  possession  of  exceptionally 
sluggish  habits ;  and  the  extreme  mobility  of 
the  eyes  compensates  for  the  relative  inflexibility 
of  the  body.  Nor  is  there  any  correlation 
between  the  oviposition  of  the  Platypus  (Ornitho- 
rhynchus)  and  the  shape  of  its  bill ;  the  latter  is 
correlated  with  its  aquatic  and  feeding  habits. 
The  conjunction  of  characters  may  be  a  coin- 
cidence ;  the  repetition  of  the  coincidence  is  an 
act  of  convergence.1 

1  For  a  discussion  of  correlation  between  teeth  and  limbs  in 
the  evolution  of  Mammalia  see  Professor  H.  F.  Osborn's  paper 
on  "The  Law  of  Adaptive  Radiation,"  American  Naturalist^ 
vol.  xxxvi.,  1902,  pp.  353-363. 


CHAPTER  VI 

DIVERGENCE   AND    PARALLELISM 

FEW  things  are  more  astonishing  at  first  acquaint- 
ance than  examples  of  close  convergence,  whether 
mimetic  or  homoplastic,  because  they  appear  para- 
doxical and  contrary  to  expectation.  In  fact  the 
ways  of  convergence  are  devious,  many-hued, 
illuminating,  full  of  surprises,  inviting  away  from 
the  narrow  path  of  homology  which,  at  least  in 
extreme  cases,  begins  and  ends  in  obscurity.  It 
must,  however,  be  acknowledged  that  the  pioneer 
work  which  effected  the  discovery  of  the  pelagic 
larvae  of  so  many  marine  animals  about  the 
middle  of  the  nineteenth  century,  was  thoroughly 
satisfactory,  and  seemed  to  indicate  to  a  later 
generation  that  the  pursuit  of  homology  as  an 
aid  to  phylogeny  was  not  wholly  elusive.  If 
there  has  been  any  mistake  it  has  been  that  of 
not  defining  the  different  degrees  of  homology. 
But  there  is  no  necessity  to  dwell  upon  mistakes 
nor  to  regret  them. 

As  a  seeming  paradox  there  comes  the  pre- 
liminary axiom  that  convergence  depends  firstly 
on  divergence  and  secondly  on  parallelism.  As 

69  E  2 


DIVERGENCE  AND   PARALLELISM 


we  ascend  higher  in  the  scale  of  nature  we  find 
that  the  co-ordinating  mechanism  of  the  animal 
body  keeps  pace  with  the  rest  of  the  organisation, 
and  the  leading  members  of  the  several  phyla  or 
branches  of  the  animal  kingdom  become  ever 
farther  removed  from  each  other  and  from  what 


FlG.  2.     Diagram  of  phyletic  divergence  and  parallelism. 

we  may  designate  vaguely  as  their  common 
starting-point.  Thus  the  highest  Mollusca,  the 
Cephalopoda  (squids,  octopods,  cuttle-fishes,  and 
Nautilus),  which  are  characterised  by  a  high 
degree  of  cephalisation  or  concentration  and 


PARALLEL  DIVISIONS  71 

amalgamation  of  parts  to  form  the  head,  repre- 
sent the  maximum  organic  development  within 
the  Molluscan  phylum,  and  are  therefore  farther 
removed  from  the  highest  mammals  than  is  the 
lowly  Chiton,  because  the  Mollusca  and  the 
Vertebrata  have  been  advancing  along  inde- 
pendent parallel  lines,  and  the  cephalisation,  with 
its  implied  brain-power,  is  an  act  of  convergence. 


FIG.  3.     Parallel  divisions  of  Echinodermata. 

The  Molluscan  and  Vertebrate  phyla,  and 
all  the  phyla,  although  their  actual  origins  are 
lost  in  Silurian  darkness,  may  or  must  be  sup- 
posed to  have  diverged  from  a  common  base 
in  the  first  instance.  This  point  is  illustrated 
graphically  in  Fig.  2  ;  and  the  same  principle  of 
primary  divergence  and  secondary  parallelism  will 
apply  to  each  phylum  (Figs.  3  and  4).  Nothing 


72  DIVERGENCE   AND   PARALLELISM 

in  phylogeny  seems  more  firmly  established 
than  the  pelmatozoic  ancestry  of  the  Echino- 
derms  (star-fishes,  sea-urchins,  etc.),  nor  than 
the  bilaterally  symmetrical  ancestry  of  the 
Pelmatozoa,  the  former  deduction  largely  result- 
ing from  their  paleontology,  the  latter  confirmed 
by  their  embryology.1 

As  for  the  Vertebrata,  not  all  of  them  possess 
vertebrae,  but  all  have  a  notochord.     The  term 


/n. 


FIG.  4.    Parallel  divisions  of  Vertebrata. 

Protochordata,  with  the  subsidiary  terms  Uro- 
chordata  (for  the  Tunicata  or  Ascidians)  and 
Cephalochordata  (for  the  Acrania  or  Amphioxus), 
was  introduced  by  Balfour  in  his  "Comparative 
Embryology"  in  1882.  Subsequently  in  1884 
Bateson  suggested  the  additional  term  Hemi- 
chordata  for  the  Enteropneusta  ("  Balano- 


1  C/.E.W.  MacBride,  "  Echinodermata  "  in  Cambridge  Natural 
History,  vol.  i.,  1906. 


MECHANISM  OF  NUTRITION  73 

glossus  ").  Dr  Gaskell l  has  reversed  the  position 
by  the  recent  statement  that  a  large  number 
of  zoologists,  who  consider  the  notochord  as  a 
great  characteristic  of  the  vertebrate  organisa- 
tion, have  followed  Bateson  in  the  matter  of 
terminology. 

Taken  as  a  whole  the  habits  of  animals  are 
cast  into  certain  well-defined  moulds,  and  it  is 
quite  easy  to  understand  why  this  should  be 
so ;  but  the  much  more  apparent  than  real 
simplicity  of  the  matter  is  no  reason  why  it 
should  not  be  stated  at  length.  Animals  differ 
from  plants  in  their  nutrition,  mobility,  and 
sensibility.  The  manner  in  which  the  nutrition 
of  animals  differs  from  that  of  green  plants  is 
sharp  and  unequivocal,  so  much  so  that  it  can 
be  expressed  in  two  contrasting  terms,  holozoic 
and  holophytic,  between  which  extremes,  how- 
ever, all  grades  of  what  is  known  as  mixotrophism 
(mixed  dietary)  are  to  be  found,  chiefly  but 
not  entirely  amongst  the  lower  orders. 

Green  plants  alone  can  elaborate  their  own 
food,  with  the  help  of  the  sun,  from  inorganic 
substances ;  animals  require  their  food  ready 
made  before  they  can  begin  to  assimilate  it. 
Plants  have  only  to  assimilate,  and  that  in  one 
way ;  animals  have  first  to  procure  their  food, 
and  that  in  many  ways.  And  yet  there  is  as 

1  W.  H.  Gaskell,  "Origin  of  Vertebrates,"   London,  1908,  set 
p.  1 6. 


74  DIVERGENCE   AND   PARALLELISM 

much  relative  diversity  amongst  plants  as 
amongst  animals,  showing  that  differentiation 
proceeds  from  within  though  it  may  be  modified 
from  without.  The  methods  adopted  for  securing 
food  determine  in  great  measure  the  habits  of 
animals.  Their  instincts  and  intelligence  are 
concentrated  upon  their  food-supply,  reproduc- 
tion being  automatic  and  all  besides  incidental. 
It  is  small  wonder  then  that  many  animals 
follow  similar  methods  ;  and  in  this  way  habitual 
convergence  is  brought  about.  Thus  the  car- 
nivorous, insectivorous,  herbivorous,  and  fru- 
givorous  habits  are  widely  disseminated ;  and 
community  of  habit  and  experience  is  often 
accompanied  by  greater  or  less  similarity  of 
structure  of  certain  parts,  giving  rise  to  a  two- 
fold exhibition  of  convergence,  bionomical  and 
homoplastic. 

Next  to  the  function  of  nutrition  we  have  to 
consider  the  organs  which  perform  that  function, 
i.e.,  the  mechanism  of  nutrition.  For  the  sake  of 
brevity  we  may  concentrate  our  attention  upon 
the  higher  or  leaf-bearing  plants  and  the  higher 
or  gut-bearing  animals.  Now  some  parasitic 
plants,  the  so-called  root-parasites  (Balanophora, 
Rafflesia),  can  manage  without  green  leaves, 
and  some  parasitic  animals,  both  endoparasites 
(Cestoda  and  Acanthocephala)  and  ectoparasites 
(Sacculina),  can  dispense  with  a  gut.  When, 
however,  as  in  the  great  majority  of  vascular 


HOMOLOGY  OF  THE  GUT  75 

plants,  true  foliage  leaves  occur,  they  are  homo- 
logous throughout.  Sometimes  other  structures 
(cladodes  and  phyllodes)  can  function  as  leaves, 
i.e.,  as  organs  of  assimilation ;  but  the  special 
homologies  of  certain  leaf-like  structures  do  not 
affect  the  general  homology  of  leaves,  which  is 
one  of  the  cardinal  points  in  vegetable  mor- 
phology. The  distinction  between  the  special 
homology  of  parts  and  the  general  homology 
of  the  whole  requires  to  be  emphasised. 

The  special  morphology  of  the  alimentary  tract 
of  animals  is  a  large  subject  which  demands 
exhaustive  analysis ;  but  it  is  distinct  from  the 
question  of  the  general  homology  of  the  gut.  It 
is  probable  that  no  single  division  of  an  insect's 
gut,  for  example,  can  be  strictly  homologised 
with  a  corresponding  part  of  a  fish's  gut,  but 
looked  at  in  their  entirety,  the  general  homology 
of  the  one  with  the  other  is  still  a  question 
apart,  to  be  answered  by  an  appeal  to  first  prin- 
ciples. That  it  must  be  answered  one  way  or 
the  other  is  evident  from  the  fact  that  objections 
have  been  raised  against  the  alleged  view  "that 
the  one  organ  which  is  homologous  throughout 
the  animal  kingdom  is  the  gut,"  [Gaskell,  /.£.]. 
This  is  perhaps  an  extreme  way  of  putting  it, 
but  we  may  examine  the  point  very  briefly, 
always  bearing  in  mind  the  distinction  between 
special  and  general  homology. 

Dr   Gaskell   regards    the    nervous   system   as 


76  DIVERGENCE   AND   PARALLELISM 

constituting  the  master  tissue  of  the  animal 
body,  and  there  is  no  doubt  that  he  is  right 
up  to  a  certain  point.  The  behaviour  of 
Protozoa  and  Sponges  in  this  regard,  however, 
teaches  us  that  in  the  course  of  evolution  the 
nervous  mechanism  is  secondary  as  compared 
with  the  mechanism  of  locomotion  on  the  one 
hand,  and  nutrition  on  the  other.  An  obvious 
rejoinder  to  this  argumentation  might  be  that 
the  organisation  of  Protozoa  and  Sponges  also 
teaches  us  that  the  gut,  as  an  organ,  is  secondary 
to  nutrition  as  a  function.  That  this  is  so  is 
borne  out  by  the  fact,  referred  to  above,  that 
just  as  in  plants  certain  structures  which  are  not 
leaves  have  secondarily  acquired  the  function 
of  leaves,  so  amongst  animals  the  parasitic 
Cestode  and  Acanthocephalous  worms,  which 
have  secondarily  lost  the  use  and  presence  of 
a  gut  as  a  consequence  of  their  mode  of  life, 
absorb  fluid  nutriment,  prepared  by  the  host, 
through  the  outer  surface  of  the  body.  Here 
therefore  the  body-wall  does  duty  for  the  gut- 
wall,  combining  the  somatic  and  splanchnic 
functions  into  one. 

We  are  therefore  driven  to  enquire  whether 
the  general  homology  of  the  gut  of  triploblastic 
(three-germ-layered)  animals  still  retains  its 
dominance,  like  that  of  the  foliage  leaf,  in  spite 
of  exceptional  cases  and  special  homologies. 
The  only  reply  to  this  imaginary  question  that 


CONVERGENT  METAMERISM  77 

presents  itself  to  my  mind  is  this :  —  The 
integrity  of  the  gut  throughout  the  Triplo- 
blastica  cannot  be  assailed  without  invalidat- 
ing the  continuity  of  the  archenteric  cavity  or 
primitive  gut  throughout  the  Metazoa ;  but  this 
is  to  strike  at  the  root  of  the  entire  fabric  of 
comparative  morphology,  since  the  archenteron 
is  as  much  the  palladium  of  animal  morphology 
as  the  primitive  leaf-member  is  the  root- 
conception  of  botany. 

The  Triploblastica  are  composed  of  two  great 
series  representing  distinct  phyletic  types,  namely, 
the  Platyhelminthes  and  the  Coelomata.  The 
highest  flatworms,  the  Nemertina,  show  points 
of  convergence  towards  Coelomata  just  as  the 
highest  Mollusca,  the  Cephalopoda,  show  points 
of  convergence  towards  Vertebrata. 

Inseparable  from  the  question  of  the  general 
homology  of  the  gut  in  coelomate  animals  is 
that  of  the  general  homology  of  the  coelom  or 
secondary  body-cavity  itself.  It  must  suffice  here 
to  affirm  this  general  homology,  again  on  first 
principles  ;  but  just  as  the  divisions  of  the  gut  are 
obviously  not  homologous  throughout  the  coelo- 
mate series,  neither  are  the  subdivisions  of  the 
coelom,  i.e.,  the  somites,  necessarily  homologous. 
A  phenomenon  analogous  to  metameric  segmenta- 
tion is  that  of  strobilation,  which  is  definitely 
known  to  occur  independently  in  different  phyla, 
e.g.,  Coelenterata,  Platyhelminthes,  Annelida.  In 


78  DIVERGENCE   AND   PARALLELISM 

some  Annelid  worms,  e.g.,  Syllidae,  strobilation  is 
superadded  to  the  normal  segmentation.  As  I 
have  indicated  in  the  diagram  (Fig.  2),  the 
Appendiculata  are  here  regarded  as  a  parallel 
stem  to  the  Vertebrata,  having  a  common 
Coelenterate  origin.  This  scheme  is  in  accord- 
ance with  the  conviction  that  the  somites  of 
Annelids  and  Arthropods  have  been  evolved 
along  their  own  line  of  descent,  independently 
of  the  somites  of  Vertebrata. 

The  metamerism  of  Appendiculata  and  Verte- 
brata is  therefore,  on  this  view,  a  conspicuous 
example  of  convergence,  and  as  such  I  regard 
it,  taking  into  account  the  facts  which  have  been 
made  known  concerning  it.  If  this  view  is 
correct,  as  I  believe  it  to  be,  there  can  be  no 
question  of  comparing  the  regional  differentia- 
tion of  Arthropods,  such  as  Limulus  and  the 
Scorpion,  with  that  of  Vertebrata.  The  whole 
comparison,  except  by  way  of  a  possible  con- 
vergence here  and  there,  is  ruled  out  of  court. 
The  existence  of  convergent  strobilation  is  in 
itself  presumptive  evidence  of  the  possibility  of 
convergent  metamerism,  and  there  are  other  facts 
which  render  it  extremely  probable  and,  to  my 
mind,  practically  certain. 

Instances  of  parallel  convergence  are  so 
numerous  and  so  common  that  we  begin  to 
realise  that  convergence  is  a  regular  and  not 
an  exceptional  phenomenon.  The  most  striking 


MAMMALIAN   CONVERGENCE  79 

example  of  the  three  principles  of  divergence, 
convergence  and  parallelism,  at  one  and  the 
same  time,  is  of  course  that  which  is  afforded  by 
the  parallel  series  presented  by  the  Marsupial 
Mammals  or  Metatheria,  on  the  one  hand,  and 
the  ordinary  Placental  Mammals  or  Eutheria,  on 
the  other.  I  have  compiled  a  table  and  give 
a  graphic  representation  of  it  in  Fig.  5.*  The 
inward  bends  of  the  parallel  waved  lines  indicate 
particular  features  of  convergence  selected  for 
the  purpose  of  the  diagram.  A  similar  diagram 
could  be  constructed  for  comparing  the  series  of 
Insectivora  and  Rodentia,  the  spiny  armature  of 
the  hedgehogs  approximating  to  that  of  the 
porcupines,  the  arboreal  habit  of  tree-shrews 
(Tupaiidae)  to  that  of  squirrels  (Sciuridae),  the 
terrestrial,  nocturnal,  and  semi-domesticated  habit 
of  land-shrews  to  that  of  mice  and  rats,  while 
the  aquatic  habit  and  the  parachute  flight  are 
also  met  with  in  both  orders.  The  musk-shrew, 
Crocidura  murina,  is  very  rat -like  in  general 
deportment,  although  its  eyes  are  small  and  its 
dentition  that  of  Insectivora. 

Parallel  evolution  accompanied  by  convergence 
is  the  expression  of  analogous  formations  in  two 
or  more  animals  belonging  to  different  subdivi- 
sions, which  may  have  acquired  a  similar  differ- 
entiation of  outward  appearance  or  internal 

1  The  items  are  gathered  from  Flower  and  Lydekker's  textbook 
gn  Mammals,  with  reference  also  to  Max  Weber's  "  Saiigethiere," 


8o 


DIVERGENCE   AND   PARALLELISM 


organisation  independently  along  different  lines 
of  descent,  the  points  in  which  they  resemble 
each  other  giving  no  indication  of  genetic  affinity 
nor  even  of  bionomical  association. 

The  general  resemblances  cited  above  relate 


FIG.  5.    Parallelism  with  convergence  amongst  mammals. 

M  =  Marsupials  ;  P  =  Placentals. 

This  phenomenon  was  observed  by  Cuvier  and  by  Owen.  The 
habits  which  lie  at  the  base  of  the  convergence  in  each  case  are 
indicated  by  the  numbers  : — 

1.  Carnivorous. 

2.  Ant-eating  (myrmecophagous). 

3.  Flying. 

4.  Swimming. 

5.  Burrowing  (large-eyed  forms). 

6.  Burrowing  (small-eyed  forms). 


APPENDICULATES   AND   VERTEBRATES        81 

to  the  external  configuration  of  the  body,  and 
some  of  them  at  least  are  sufficiently  remark- 
able, not  to  say  astonishing.  Perhaps  one  of 
the  closest  degrees  of  functional  convergence  is 
that  which  is  manifested  between  the  appendi- 
culate  (annelid  and  arthropod)  and  vertebrate 
central  nervous  systems,  upon  which  attention 
has  been  focussed  by  Dr  Gaskell.  The  central 
nervous  apparatus  is  the  dominant  system  in 
the  organisation  of  the  higher  animals,  and  he 
thinks  that  it  has  been  a  dominant  factor  in 
evolution.  The  comparison  tabulated  below  from 
Dr  Gaskell's  recently  published  volume  on  the 
"Origin  of  Vertebrates "( London,  1908)  appears 
to  me  to  serve  as  a  most  beautiful  example  of 
true  physiological  convergence,  but  likely  to  be 
confounded  with  the  spurious  convergence  pre- 
sented by  the  so-called  cephalic  stomach  of  the 
arthropod  in  comparison  with  the  ventricles  of 
the  brain  in  the  craniate  vertebrates.  Now,  it 
may  appear  something  like  an  impertinence  to 
describe  as  a  spurious  convergence  what  Dr 
Gaskell  regards  as  a  true  homology ;  it  is,  how- 
ever, nothing  of  the  kind,  and  is  no  more  than 
an  expression  of  the  opposite  point  of  view.  On 
that  understanding  we  may  leave  it  for  the 
present,  perhaps  returning  to  it  later,  either 
directly  or  by  implication. 


82 


DIVERGENCE   AND   PARALLELISM 


TABLE  OF  CONVERGENCE  IN  THE  STRUCTURE  OF 
THE  CENTRAL  NERVOUS  SYSTEM  (NEURAL 
CONVERGENCE) 


APPENDICULATE. 
(Segmented  invertebrate). 

1.  Supra-oesophageal  ganglia, 
giving  origin  to  the  nerves  of 
the    eyes  and  antennules,   z.e,, 
the  optic  and  olfactory  nerves. 

2.  Circum-oesophageal    com- 
missures connecting  the  cerebral 
ganglia  with  the  infra-cesopha- 
geal   ganglia   and   the   ventral 
chain. 


3.  Infra-cesophageal  ganglia 
and  the  paired  ventral  chain  of 
segmental  ganglia.  Each  pair 
of  ganglia  gives  rise  to  the 
nerves  of  its  own  segment, 
motor  and  sensory;  by  the 
agency  of  which  food  is  in- 
gested, respiration  and  loco- 
motion effected. 


VERTEBRATE. 
(Craniate  vertebrate}. 

1.  The    brain    proper    from 
which  arise  only  the  olfactory 
and  optic  nerves. 

2.  Crura  cerebri^  strands   of 
fibres  on  each  side  of  the  in- 
fundibulum,      connecting     the 
higher     brain    region     proper 
with  the  lower  region  of   the 
medulla  oblongata  and  spinal 
cord. 

3.  Region  of  the  mid-brain, 
medulla  oblongata,  and  spinal 
cord  ;  from  these  arises  a  series 
of  nerves  segmentally  arranged, 
which,  as  in  the  invertebrate, 
give     origin     to     the     nerves 
governing  mastication,  respira- 
tion, and  locomotion. 


With  reference  to  the  first  sub-heading  in  the 
above  table  it  is  to  be  remarked  that  Dr  Gaskell 
duly  points  out  that  the  crustacean  antennules 
are  olfactory  in  function,  but  he  does  not  add 
that  they  are  also  the  carriers  of  the  auditory 
organs  and  that  the  latter  are  innervated  from 
the  cerebral  ganglia  as  they  are  in  Mollusca.  At 
the  same  time  a  physiologist  may  be  pardoned 
for  regarding  the  close  correspondence  as  a  mani- 
festation of  homology  rather  than  as  a  case  of 


APPENDICULATES  AND  VERTEBRATES         83 

convergence ;  and  there  are  other  facts  which 
serve  still  more  to  complicate  the  issue  and  to 
baffle  the  most  competent  judgment.  Chief 
amongst  these  is  the  apparent  topographical 
coincidence  between  the  infundibulum  in  the 
floor  of  the  third  ventricle  of  the  vertebrate 
brain  and  the  appendiculate  oesophagus. 

If  we  admit  the  convergence  as  tabulated 
above,  why  should  we  not  regard  this  further 
coincidence  as  another  case  of  convergence  ? 
There  are  at  least  two  reasons  why  I  think  we 
should  not :  firstly,  because  the  structural  con- 
vergence in  the  two  types  of  central  nervous 
system  is  accompanied  by  a  certain  degree  of 
functional  equivalence,  whereas  the  topographical 
coincidence  of  infundibulum  and  oesophagus  is 
not  accompanied  by  any  functional  equivalence ; 
secondly,  because  the  infundibulum  can  be  brought 
into  interesting  topographical  correlation  with 
a  primitive  feature  quite  different  from  the 
appendiculate  oesophagus,  namely,  the  anterior 
neuropore  or  anterior  neurenteric  canal  of  pro- 
tochordates.  If  the  striking  correspondence  in 
plan  of  composition  of  the  central  nervous 
system  in  appendiculates  and  vertebrates  had 
been  recognised  as  an  instance  of  general 
structural  convergence  thirty  -  five  years  ago, 
when  the  late  Dr  Anton  Dohrn,  the  founder  of 
the  Stazione  Zoologica  at  Naples,  explained  his 
application  of  the  principle  of  change  of  function 


84  DIVERGENCE   AND   PARALLELISM 

in  relation  to  the  origin  of  vertebrates  (1875), 
it  would  not  have  led  to  what  many  zoologists 
are  constrained  to  regard  as  a  series  of  false 
homologies ;  and  per  contra  it  would  not  have 
led  to  many  interesting  discoveries. 

There  are  two  factors  which  conspire  to 
beguile  a  morphologist  into  spurious  relations 
and  vain  imaginings  :  these  are  the  potency  of 
convergence  and  the  weight  of  coincidence.  A 
very  eminent  French  zoologist,  who  enjoyed  the 
respect  and  veneration  of  all  who  came  directly 
or  indirectly  within  the  sphere  of  his  influence, 
the  late  Professor  Henri  de  Lacaze-Duthiers, 
thought  from  the  beginning  to  the  end  that 
the  Ascidians  (Tunicata)  are  related  to  the 
Mollusca.  In  apparent  confirmation  of  this  belief 
he  discovered  the  case  of  a  simple  Ascidian,  a 
species  of  Molgula,  which  develops  without  the 
intervention  of  the  usual  tailed  larva ;  and  then 
he  discovered  a  form  in  the  Mediterranean  which 
he  called  Chevreulius,  but  which  had  already 
been  named  Rhodosoma  from  the  Pacific,  where 
the  atrial  and  buccal  siphons  lie  within  a  hinged 
valve  which  can  be  opened  and  closed  like  the 
valves  of  a  lamellibranchiate  mollusc. 

It  is  indeed  extraordinary  to  relate,  in  Dr 
Gaskell's  words,  that  the  infundibulum  "lies  just 
anteriorly  to  the  exits  of  the  third  cranial  or 
oculomotor  nerves ;  in  other  words,  it  marks  the 
termination  of  the  series  of  spinal  and  cranial 


VERTEBRATE   DESCENT  85 

segmental  nerves.  .  .  .  Not  only,  then,  are  the 
nerve-masses  in  the  two  systems  [appendiculate 
and  vertebrate]  exactly  comparable,  but  in  the 
very  place  where  the  cesophageal  tube  is  found 
in  the  invertebrate,  the  infundibular  tube  exists 
in  the  vertebrate.  ..."  All  this  is  true,  but 
what  Dr  Gaskell  does  not  relate  at  this  crucial 
point  of  his  narrative  (/.£.,  p.  14)  is  that  the 
infundibulum  also  occurs  essentially  at  the  same 
horizon,  with  reference  to  the  segmental  nerves, 
as  the  anterior  neuropore  and  olfactory  pit  of 
Amphioxus,1  where  there  are  likewise  two 
anterior  pairs  of  sensory  cranial  nerves  ter- 
minating in  peripheral  ganglion  cells  on  the 
prseoral  lobe. 

It  is  not  particularly  edifying  to  pursue  one 
phantom  rather  than  another  ;  and  the  spirit  of 
morphology  may  be  said  to  reside  not  so  much 
in  a  desire  for  ultimate  truth,  that  is  to  say,  to 
attain  the  unattainable,  as  in  the  will  to  accept 
nothing  but  the  truth.  Applying  this  aphorism 
to  the  special  case  of  the  two  opposing  theories 
of  vertebrate  descent,  namely,  the  Appendiculate 
theory  and  the  Protochordate  theory,  we  may 
say  that  if  the  latter  fails  to  satisfy,  so  at  least 
does  the  former.  It  is  necessary  to  codify  the 
facts ;  but  this  can  be  done  for  the  present  upon 
a  broad  basis  of  convergence  better  than  by 
an  unrestrained  use  of  homology. 

1  Dr  Gaskell's  references  to  the  anterior  neuropore  of  Amphioxus 
occur  on  pp.  220  and  457  of  his  work,  to  which  I  refer  the  reader. 

F  2 


86  DIVERGENCE  AND  PARALLELISM 

Meanwhile  we  may  agree  unreservedly  with 
Dr  Gaskell's  opinion  that  nothing  but  good  can 
result  from  the  incursion  of  the  physiologist  into 
the  realm  of  the  morphologist.  Then,  and  then 
only,  do  we  begin  to  realise  how  we  stand  in 
regard  to  the  facts  of  comparative  anatomy, 
and  to  perceive  the  paramount  importance  of 
convergence. 

With  regard  to  the  special  case  of  the  nervous 
system  we  must  add  that  there  is  no  ground  for 
imagining  that  it  is  the  one  organic  system 
throughout  the  animal  kingdom  which  offers  no 
instances  of  convergence. 


CHAPTER  VII 

SPECIAL  CONVERGENCE 

SPECIAL  convergence  depends  upon  functional 
equivalence,  which  may  or  may  not  be  accom- 
panied by  partial  homology.  The  different 
degrees  may  be  expressed  in  various  ways  at 
convenience,  according  as  it  is  desired  to  focus 
attention  upon  a  function  or  an  organ,  or  upon 
the  systematic  position.  In  the  systematic 
method  of  arrangement  one  makes  use  of  the 
terms  employed  in  classification.  Individual  or 
racial  convergence  refers  to  those  resemblances 
between  varieties  of  a  species  such  as  are 
familiar  in  the  human  race  as  the  so-called 
"  doubles."1  Specific,  generic,  family,  ordinal, 
class,  and  phyletic  convergence  refer  respectively 
to  the  species  of  a  polytypic  genus,  to  the 

1  Since  the  above  was  written  Professor  Adam  Sedgwick  has 
kindly  drawn  my  attention  to  a  paper  by  Professor  Arthur  Keith  on 
"  The  Position  of  the  Negro  and  Pygmy  amongst  Human  Races"  in 
Nature^  vol.  Ixxxiv.,  1910,  p.54.  Professor  Keith  thinks  that  pygmies, 
who  are  widely  distributed,  are  "modifications  produced  locally  from 
the  larger  negro.  .  .  .  The  Congo  pygmies  share  all  the  physical 
features  of  the  Bantu  except  size  ;  the  Bushman  has  the  characters 
of  the  Hottentot,  while  the  pygmies  of  the  Far  East  find  their  nearest 
representatives  in  the  negroes  of  Oceania." 


88  SPECIAL  CONVERGENCE 

genera  of  a  family,  to  the  families  of  an  order, 
to  the  orders  of  a  class,  to  the  classes  of  a 
phylum  and  to  the  phyla  of  the  animal  kingdom. 
One  of  the  best  examples  of  special  family 
convergence  is  afforded  by  the  pectoral  fins  of 
flying  fishes.  Semper  (pp.  cit.)  dealt  with  the 
organs  of  flight  in  Vertebrata,  and  figured  a 
flying  "  herring,"  Exoccetus,  belonging  to  the 
Teleostean  family  Scombresocidse,  in  juxta- 
position with  a  bat ;  but  he  did  not  refer  to 
the  case  of  the  flying  gurnard,  Dactylopterus, 
which  belongs  to  another  family,  the  Triglidse 
(sometimes  called  Cottidae  and  Cataphracti). 
Both  of  these  genera  occur  in  the  Mediter- 
ranean as  well  as  in  the  Indian  Ocean,  and 
are  totally  different  from  each  other,  not  only 
in  systematic  position  but  in  external  form. 
Dactylopterus  has  a  broad,  depressed  head, 
armed  with  powerful  spines,  on  account  of 
which  the  head  is  commonly  fractured  by  a 
blow  when  the  fish  is  caught  by  native  fisher- 
men off  the  coast  of  Ceylon ;  the  scales  are 
hard,  keeled  scutes ;  the  tail  -  fin  is  truncated, 
the  abdomen  flattened,  and  the  body  coloured 
red.  Exoc&tus  has  a  normal  herring-like  head 
and  body,  unarmed,  with  smooth  scales,  a  deeply- 
forked  tail-fin,  a  convex  abdomen,  and  a  silvery 
ground  colour.  Both  kinds  of  flying-fishes  owe 
their  limited  power  of  flight  above  the  surface 
of  the  sea  to  the  secondary  elongation  and 


WING  FLIGHT  89 

expansion  of  the  pectoral  fins,  an  exceptional 
modification  which  has  been  acquired  inde- 
pendently within  the  limits  of  two  very  distinct 
families. 

It  is  remarkable  to  find  such  strictly  homo- 
logous organs  as  the  pectoral  fins  of  Teleostean 
fishes  modified  in  a  virtually  identical  manner 
to  perform  a  special  and  exceptional  function, 
whose  transformation  is  nevertheless  not  homo- 
genetic but  homoplastic.  This  typical  example 
serves  to  illustrate  one  of  the  most  interesting 
manifestations  of  convergence,  namely,  the  homo- 
plastic  modification  of  homogenetic  structures. 

The  comparison  between  the  wings  of  birds 
and  of  bats  offers  different  conditions.  They  are 
equally  efficient  as  organs  of  sustained  aerial 
flight ;  they  show  complete  functional  equiva- 
lence ;  the  fore-limbs  are  homologous ;  but  the 
transformation  has  proceeded  along  divergent 
lines,  an  expanded  wing-membrane  or  patagium 
with  reduced  epidermal  appendages  (hairs)  in  the 
one  case ;  a  rudimentary  patagium  and  enlarged 
epidermal  appendages  (feathers)  in  the  other;  with 
corresponding  differences  affecting  the  skeleton  of 
the  wing-supporting  limbs,  hypertrophied  digits 
in  the  bat,  atrophied  digits  in  the  bird.  Unlike 
the  flying  fishes,  the  modification  has  not  taken 
place  in  an  identical  manner,  but  in  an  almost 
diametrically  opposite  manner ;  it  cannot  there- 
fore be  described  as  homoplastic  in  the  strict 


90  SPECIAL  CONVERGENCE 

sense  of  the  term,  although  it  is  frequently  quoted 
as  a  stereotyped  example  of  homoplasy.  Perhaps 
it  would  be  more  logical  to  express  it  in  other 
language  for  which  a  suitable  technical  term  does 
not  seem  to  be  available,  unless  we  may  borrow 
a  word  from  another  division  of  the  subject  and 
call  it  a  heterotypic  modification  of  homogenetic 
structures,  the  common  starting-point  being  the 
pentadactyle  limb  of  terrestrial  vertebrates. 

Birds  and  bats  are  directly  comparable  in  terms 
of  the  theory  of  convergence,  not  only  because 
they  are  alone  amongst  vertebrate  animals  in 
their  capacity  for  rapid  and  changing  flight,  but 
also  because  they  are  both  warm-blooded.  Fly- 
ing fishes  bear  the  same  relation  to  birds  as  they 
do  to  bats,  whatever  that  relation  may  be  held 
to  be.  In  my  opinion  they  are  in  no  way  com- 
parable, inasmuch  as  the  flight  of  fishes  is  of  the 
nature  of  a  parachute  flight  and  so  belongs  to  a 
different  category.  Although  well  enough  twenty 
years  ago,  it  would  not  be  admissible  to-day  to 
figure  a  flying  fish  and  a  bat  upon  the  same  page 
for  the  purpose  of  illustrating  the  principle  of 
homoplastic  wing-formation,  except  in  respect  of 
the  one  point  of  the  hypertrophy  of  the  pectoral 
rays  and  digits.  It  would  be  more  appropriate 
to  present  a  flying  fish  in  conjunction  with  a 
flying  lizard,  or  even  a  flying  squirrel,  the  nature 
of  the  relation  between  them  being  one  of  general 
functional  convergence,  neither  associated  with 


PARACHUTE   FLIGHT  91 

homology  nor  with  general  structural  convergence 
or  homoplasy. 

The  case  of  Pteromys  and  Galeopithecus, 
quoted  above,  may  be  regarded  as  an  example 
of  the  homoplastic  modification  of  homogenetic 
structures  in  different  orders,  i.e.,  ordinal  con- 
vergence ;  but  the  homoplasy,  in  the  literal  sense, 
is  not  complete,  since  there  are  important  differ- 
ences of  structural  detail,  the  digits  of  Galeopithecus 
being  webbed,1  and  the  wrist  of  Pteromys  carry- 
ing an  accessory  cartilaginous  rod  which  gives 
material  support  to  the  parachute. 

The  degrees  of  convergence  are  endless  and 
can  only  be  classified  in  a  broad  way ;  it  is,  we 
may  repeat,  a  fundamental  phenomenon  and  con- 
sequently of  frequent  occurrence,  though  not 
always  equally  demonstrative.  Very  striking  are 
those  cases  where  special  features  which  are 
exceptional  within  a  certain  limited  systematic 
range  are  repeated  independently,  as  the  phos- 
phorescent and  electric  organs  of  fishes.  The 
late  Professor  Howes,  in  one  of  his  addresses, 
quoted  as  a  critical  example  of  convergence  the 
fact  that  the  Mesozoic  reptiles  yielded  terrestrial, 
aquatic,  and  aerial  forms,  just  as  did  the  Tertiary 
mammals  which  replaced  the  former  in  order  of 
dominance. 

Amongst    animals   which    are    more    or    less 

1  Flower  and  Lydekker,  "Mammals  Living  and  Extinct," 
London,  1891,  Fig.  on  p.  615. 


92  SPECIAL   CONVERGENCE 

nocturnal  in  their  habits,  the  pupil  of  the  eye 
becomes  contracted  in  a  bright  light  to  a  narrow 
aperture  which  may  retain  its  normal  and  primitive 
circular  shape  or  may  become  a  vertical  ellipse 
or  a  narrow  vertical  slit  or  a  horizontal  ellipse  or 
slit.  The  pupil  of  the  cat's  eye  is  round  at  night 
time,  vertically  contracted  during  the  day  ;  the 
leopard's  eye  and  the  jaguar's,  on  the  contrary, 
like  the  lion  and  tiger,  contract  in  bright  sun- 
light to  a  circular  pinhole  aperture.  Mr  Frank 
Buckland  has  noted  that  the  pupil  of  the  fox's 
eye  behaves,  not  like  the  dog's,  as  one  would 
expect,  but  like  the  cat's,  and  this  linear  pupil  is, 
by  some  writers,1  advanced  as  a  generic  character 
of  the  genus  Vulpes,  as  distinguished  from  Cam's. 
Amongst  tailless  Batrachia  (frogs  and  toads) 
the  most  extensive  genera,  Rana,  Bufo,  Rhaco- 
phorus  and  Ixalus,  have  a  horizontal  pupil,  a 
vertical  pupil  being  the  exception  in  this  division. 
Geckoes  (Geckonidse),  the  lizards  of  tropical  and 
subtropical  countries  which  can  run  along  upright 
and  over  -  hanging  surfaces  by  virtue  of  the 
adhesive  pads  on  their  digits,  have  the  vertical 
form  of  pupil  ;  when  partially  contracted  the 
margin  of  the  iris  in  some  species  is  crenulated  ; 
and  when  quite  contracted  so  that  the  anterior 
and  posterior  borders  are  in  contact,  leaving  only 
a  vertical  pupillary  suture  between  them,  the 


.%.)  E.  T.  Seton,  "  Life-Histories  of  Northern  Animals."    New 
York,  1909,  2  vols. 


PUPIL   OF   THE   EYE  93 

latter  has  a  beaded  appearance,  the  beading 
corresponding  with  the  previously  noted  crenula- 
tions.  Crocodiles  also  show  the  narrow  vertical 
pupil. 

Among  snakes,  round  and  vertical  pupils  occur 
with  nearly  equal  frequency,  the  former  rather 
more  frequently  than  the  latter;  but  Dryophis, 
called  the  whip-snake,  has  a  very  pronounced 
horizontal  pupil  (Fig.  6). 

FIG.  6. 


Head  of  Dry  aphis  pulverulentus  in  side  view. 

In  the  class  Mammalia  the  round  pupil  pre- 
vails, whereas  the  vertical  pupil,  as  mentioned 
above,  characterises  certain  groups.  The  oblong 
horizontal  pupil  of  the  horse,  camel,  ox,  and  sheep 
was  referred  to  by  Owen  in  his  "  Anatomy  of 
Vertebrates"  (1866);  it  is  very  sharply  defined 
in  goats  that  have  a  gilt-edged  iris.  Owen  also 
stated  that  in  a  dead  Agouti  (Rodentia)  the 
pupil  was  a  horizontal  ellipse ;  but  he  did  not 
allude  to  the  horizontal  pupil  of  the  mungoose, 
nor  have  I  seen  it  mentioned  in  any  other  work 
so  far  as  I  can  remember.  In  the  Civet  family 
(Viverridse),  which  is  closely  allied  to  the  Felidae, 
the  vertical  pupil  appears  to  be  the  rule,  but 


94  SPECIAL   CONVERGENCE 

Herpestes,  the  Indian  mungoose,  has  a  well- 
defined,  horizontal  pupil  in  the  daytime,  becoming 
round  at  night  (Fig.  7,  Frontispiece).  The  allied 
genus  Suricata,  the  South  African  meerkat,  has 
a  similarly  shaped  pupil,  shown  very  clearly  in 
the  specimens  at  the  London  Zoological  Gardens. 

Hubrecht  (I.e.,  1908)  has  given  reasons  for 
believing  that  the  cartilaginous  fishes  occupy  a 
side  branch  of  the  vertebrate  stem,  and  he 
goes  so  far  as  to  suggest  a  radical  change  in 
the  classification  of  vertebrates,  dividing  them 
into  four  super-classes  :  Cephalochordata  (Amphi- 
oxus\ Cyclostomata  (lampreys,  etc.),  Chondrophora 
(sharks,  etc.),  and  Osteophora  (bony  fishes  and 
higher  vertebrates).  In  view  of  this  weighty 
opinion,  the  a  priori  probability  that  the  nicti- 
tating membrane  of  some  sharks  (Carchariidse), 
which  is  an  accessory  protection  for  the  eye, 
sweeping  over  the  surface  of  the  eyeball,  has 
no  genetic  connection  with  the  functionally 
equivalent  membrane  of  many  of  the  higher 
vertebrates,  is  greatly  strengthened. 

Bony  fishes  have  no  movable  eyelids,  but 
transparent  adipose  eyelids  are  sometimes  present, 
composed  of  an  anterior  and  posterior  membrane 
or  of  a  continuous  circular  membrane  perforated 
at  or  about  the  centre.  Both  the  Grey  Mullet 
family  (Mugilidse)  and  the  Herring  family 
(Clupeidae)  are  respectively  differentiated  into 
groups  by  the  presence  or  absence  of  adipose 


EYELIDS  95 

eyelids  which  must  have  arisen  independently 
in  those  families.  Where  there  is  a  continuous 
perforated  membrane,  as  in  some  Clupeidse 
(e.g.,  Dussumieria,  Fig.  8),  the  condition  pre- 
sented calls  to  mind  that  of  the  Oegopsid  cuttle- 
fishes, where  the  outer  covering  of  the  eye  is 
likewise  perforated.  No  doubt  the  two  condi- 
tions are  physiologically  comparable. 


FIG.  8.  Eye  of  Dussumieria  showing  perforated  adipose  eyelid. 
The  outline  of  the  pupil  is  seen  below  the  transparent  [dotted] 
adipose  membrane. 

Raptorial  or  prehensile  appendages  amongst 
invertebrate  animals  offer  further  analogies. 
Chelate  extremities  occur  alike  in  Crustacea 
and  Arachnida ;  and  the  characteristic  clasp 
knife  appendages  of  the  Stomatopod  Crustacea 
(Squilla)  are  found  again  in  Orthopterous  insects 
(Mantodea)  and  in  Neuropterous  insects  (Mantis- 
pidse).  In  Chiromantis,  a  tropical  African  genus 
of  frogs  allied  to  Rhacophorus,  the  two  inner 
fingers  of  the  hand  are  opposite  and  opposable 
to  the  two  outer  fingers,  so  that  its  structure  re- 
sembles that  of  the  grasping  hand  of  chameleons.1 
All  these  cases  belong  to  the  great  category 
of  opposable  extremities,  some  of  which  are 

1  G.  A.  Boulenger,  Catal.  Batr.  Sal,  1882,  p.  92,  pi.  x.,  Figs.  I 
and  2. 


96  SPECIAL  CONVERGENCE 

reciprocally  homologous,  whilst   others,   like  the 
above,  are  homoplastic. 

Another  case  worth  quoting,  as  well  known  to 
students  of  osteology  as  it  is  remarkable,  is  that 
of  convergent  opisthocoely  already  mentioned  by 
Owen l  as  occurring  in  the  North  American  fresh- 
water Ganoid  fish,  Lepidosteus  the  Garpike,  in 
the  Surinam  toad,  Pipa  americana,  and  in  the 
Salamanders.  In  Batrachia  and  Reptilia  the 
vertebrae  articulate  together  by  a  cup-and-ball 
or  ball-and-socket  arrangement,  and,  as  a  general 
rule,  the  cup  lies  at  the  front  end  of  the  centrum 
of  each  vertebra  to  receive  the  condyle  which 
projects  from  the  hinder  end  of  the  preceding 
vertebra.  Such  vertebrae  with  the  concavities  in 
front  are  called  procoelous.  In  rare  cases  the 
reversed  condition  obtains,  where  the  condyles 
project  at  the  front  ends  of  the  vertebrae  and  fit 
into  the  cups  at  the  hinder  ends  of  the  preceding 
vertebrae.  Such  vertebrae  with  the  concavities 
behind  are  called  opisthocoelous. 

The  vertebrae  of  fishes  are  joined  together  by 
ligament,  and  the  centra  do  not  possess  articu- 
lar surfaces  ;  both  in  front  and  behind  there  is 
a  deep  cup,  whence  the  vertebrae  are  designated 
amphicoelous  or  biconcave.  Lepidosteus  is  unique 
amongst  existing  fishes  in  having  opisthocoelous 
vertebrae.  The  same  description  of  vertebrae 
only  occurs  otherwise  in  certain  Batrachia,  in- 

1  R.  Owen,  "Anatomy  of  Vertebrates,"  London,  1866, 1.  pp.  33-34. 


OPISTHOCOELOUS   VERTEBRA  97 

eluding  the  newts  and  salamanders  among  the 
Urodela,  and  a  number  of  genera  of  Anura  of 
peculiar  habits  belonging  to  separate  families 
and  each  containing  a  single  species,  namely, 
Bombinator  the  European  Fire-bellied  toad,  Pipa 
the  Surinam  toad,  Alytes  the  Midwife  toad,  and 
Discoglossus,  the  Painted  frog,  a  circum-Medit- 
erranean  batrachian.  The  opisthocoely  of  Pipa 
is  probably  independent  of  that  of  the  other 
named  Anura ;  that  of  the  named  Anura  collec- 
tively is  probably  independent  of  salamandrine 
opisthocoely ;  and  Batrachian  opisthocoely  as  a 
whole  is  independent  of  that  of  Lepidosteus. 

Somewhat  analogous  to  the  reversed  articu- 
lation of  vertebrae  described  in  the  preceding 
paragraph  is  the  reversed  spire  of  the  shell  in 
Molluscs  and  the  reversed  pose  of  the  body  in 
Flatfishes.  These  two  latter  cases  of  situs  inver- 
sus  were  quoted  by  Bateson  as  examples  of 
discontinuity  in  substantive  variation,  but  they 
must  be  mentioned  again  here  as  examples  of 
convergent  variation.  "  In  both  of  the  groups 
named,  some  species  are  normally  right-handed, 
others  being  normally  left-handed,  while  as  indi- 
vidual variations  reversed  examples  are  found. 
.  .  .  The  fact  that  the  reversed  condition  may 
become  a  character  of  an  established  race  is 
familiar  in  the  case  of  Fiisus  antiqmts.  This 
shell  is  found  in  abundance  as  a  fossil  of  the 
Norwich  Crag,  such  specimens  being  normally 

G 


98  SPECIAL  CONVERGENCE 

left-handed,  though  the  same  species  at  the 
present  day  is  a  right-handed  one."1  In  India 
and  Ceylon,  where  the  chank  shell  (Turbinella 
pyruni)  plays  such  a  large  part  in  religious 
ceremonies,  the  rare  apparition  of  a  sinistral 
form  is  regarded  as  a  favourable  portent,  and 
such  a  shell  is  very  highly  esteemed.  There 
would  appear  to  be  some  common  causes 
operating  during  the  early  development  to 
procure  such  cases  of  reversal.  A  spiral  shell 
can  only  be  twisted  in  one  of  two  directions ; 
and  a  Pleuronectid  fish  can  only  lie  on  one  of 
two  sides.  These  alternative  conditions,  one  of 
which  is  frequently  the  rule,  the  other  the 
exception,  afford  an  interesting  sidelight  upon 
the  incidence  of  convergence. 

The  classification  of  reversed  molluscs2  is  the 
same  in  principle  as  that  for  reversed  flatfishes, 
namely : — 

(1)  Cases   in  which   the    genus   is   normally 

sinistral. 

(2)  Cases   in  which    the   genus   is   normally 

dextral   but   certain   species  are  nor- 
mally sinistral. 

(3)  Cases  in  which  the  species  is  indifferently 

dextral  or  sinistral. 

(4)  Cases   in   which   a   sinistral    form    is   an 

abnormal  monstrosity. 

1  W.  Bateson,  "  Materials  for  the  Study  of  Variation,"  London, 

1894,  p.  54. 

2  Cf.   A.  H.  Cooke    "Molluscs,"  Cambridge  Natural  History, 

1895,  p.  249. 


TEN-LEGGED   PANTOPODS  99 

A  striking  example  of  generic  convergence  is 
exhibited  in  the  ten-legged  Pantopods  or  "sea- 
spiders  "  which  have  been  obtained  from  the 
South  Polar  region  by  the  English  and  Scottish 
National  Antarctic  Expeditions.  They  belong 
to  two  quite  distinct  genera,  Pentanymphon 
Hodgson,  1904,  and  Decolopoda  Eights,  1837; 
the  former  only  differing  from  Nymphon  by  its 
possession  of  an  extra  pair  of  legs ;  the  latter 
most  nearly  related  to  Colossendeis,  from  which 
it  differs  likewise  by  its  possession  of  an  extra 
pair  of  legs,  and  also  by  the  presence  of  a 
pair  of  well-developed,  three-jointed  mandibles. 
These  two  Antarctic  genera  are  thus  nearly 
related  to  two  other  representative  Pantopod 
genera  but  not  to  each  other.  Professor  D'Arcy 
Thompson1  places  them  in  widely  separated 
families  :  Decolopodidae  and  Nymphonidse ;  and 
Schimkewitsch 2  has  arrived  at  the  same  con- 
clusion, namely,  that  the  ten-legged  Pantopods 
do  not  constitute  a  distinct  group  in  contrast 
with  the  more  usual  eight-legged  forms,  but 
that  they  arose  independently  from  eight-legged 
forms,  the  number  of  legs  giving  no  clue  to 
their  affinities. 

The  following  table,  compiled  from  the  data 
furnished  by  Hodgson,3  who  collected  and 

1  D'Arcy  W.  Thompson,  "  Pycnogonida,"  Camb.  Nat.  Hist.,  iv., 
1909,  pp.  531  and  537. 

2  W.  Schimkewitsch,  "  Uber  die  Periodicitat  in  dem  System  der 
Pantopoden,"  Zool.  Anz.^  xxx.,  1906,  p.  3. 

3  Besides  the  Reports  of  the  Voyages,  see  the  following  papers 


ioo  SPECIAL   CONVERGENCE 

described  Pentanymphon  during  the  National 
Antarctic  Expedition  in  the  Discovery,  and 
redescribed  Decolopoda  from  material  procured 
by  Mr  W.  S.  Bruce  during  the  Scottish  National 
Antarctic  Expedition  in  the  Scotia,  shows  the 
radical  differences  which  exist  between  the  two 
genera  simultaneously  with  the  numerical  con- 
vergence of  the  ambulatory  appendages.  The 
differential  characters  indicate  the  respective 
places  of  the  genera  in  the  system  of  Pantopoda  ; 
the  convergent  character  gives  no  indication  of 
their  affinities  to  any  existing  forms. 

TABLE   OF   CONVERGENCE   IN   PANTOPODA. 

PENTANYMPHON.  DECOLOPODA. 

1.  Ambulatory  appendages,          Ambulatory  appendages,  five 
five  pairs.  pairs. 

2.  Body     elongate,     clearly          Body   short,    obscurely    seg- 
segmented,   very  slender,  with  mented,   broadly    elliptical    in- 
lateral  processes  widely  separ-  eluding  the  lateral  processes  in 
ated.  the  contour  of  the  ellipse. 

3.  Proboscis    cylindrical,          Proboscis  clavate,  bent  down- 
straight,  much  shorter  than  the      wards,  longer  than  the  body, 
body. 

4.  Mandibles    chelate,    2-          Mandibles  chelate,  3-jointed. 
jointed. 

5.  Palps  5-jointed.  Palps  lo-jointed. 

6.  Ovigerous  legs,  lo-jointed,          Ovigerous   legs,    10- jointed, 
formed  plainly  as  in  Nymphon.      looped  as  in  Colossendeis. 

7.  Abdomen     small,     ovoid,          Abdomen  long,  slender,  sub- 
directed  obliquely  upwards.  clavate. 

by  T.  V.  Hodgson  :  I.  "  On  a  New  Pycnogonid  from  the  South 
Polar  Regions,"  Ann.  Nat.  Hist.  (7),  xiv.,  1904,  pp.  458-462,  pi. 
xiv.  2.  "Scotia  Collections.  On  Decalopoda  australis  Eights, 
an  old  Pycnogonid  rediscovered,"  Proc.  Roy.  Phys.  Soc.,  Edin- 
burgh, vol.  xvi.,  1905,  pp.  35-42,  pi.  iii.  3.  "  Decalopoda  and 
Colossendeis,"  Zool.  Anz.,  xxix.,  1905,  pp.  254-256.  [The  accepted 
spelling  of  the  former  name  is  now  Decolopoda.] 


TEN-LEGGED   PANTOPODS  101 

This  table  shows  two  genera  belonging  to 
different  families  of  a  comparatively  small  order, 
with  one  very  prominent  character  in  common 
which  cannot  be  referred  back  to  a  common 
origin. 

Quite  recently  a  letter  from  Dr  W.  T.  Caiman, 
containing  further  references  to  Antarctic  Panto- 
poda,  appeared  in  Nature  (vol.  Ixxxiv.,  28th  July 
1910,  p.  104),  which  adds  to  the  importance  of 
this  case.  A  third  genus  of  ten-legged  Panto- 
pods,  named  Pentapycnon  by  Professor  E.  L. 
Bouvier,  has  been  obtained  by  Dr  Charcot's 
expedition  in  the  Pourquoi  Pas,  and  "strange 
to  say,  it  appears  to  be  quite  unrelated  to  the 
other  two.  Pentapycnon  charcoti  is  a  near  relative 
of  Pycnogonum,  hitherto  regarded  as  the  most 
highly-specialised  of  all  Pycnogonida.  Further, 
just  as  Pentanymphon  is  accompanied  by  a  species 
of  Nymphon,  and  Decolopoda  by  Colossendeis,  so 
Professor  Bouvier  finds  that  Pycnogonum>  hitherto 
unknown  from  Antarctic  seas,  is  represented  by 
a  new  species  alongside  of  Pentapycnon  at  the 
South  Shetlands." 

We  may  now  consider  the  case  of  two  genera 
of  toads  belonging  to  one  family  (Bufonidae)  of 
a  large  order  (Batrachia-Anura).  These  show 
several  exceptional  characters  in  common,  side 
by  side  with  a  number  of  strongly  divergent 
features.  The  divergences  in  structure  and 
distribution  are  so  great  as  to  make  it  doubtful 

G  2 


102  SPECIAL  CONVERGENCE 

whether  any  of  the  other  characters  are  trace- 
able to  direct  genetic  affinity ;  and  Gadow 1 
suggests  that  their  community  of  habits  with 
concomitant  modifications  of  structure  .may  be 
due  to  convergent  evolution.  The  structural 
features  in  the  table  given  below  are  taken  from 
Boulenger ; 2  the  feeding  habits  are  gathered 
from  Gadow.  Both  of  these  authors  note  their 
resemblance  to  the  firmisternal  family  of  Engy- 
stomatidae,  on  the  one  hand,  and  their  systematic 
position  under  the  arciferous  Bufonidae,  on  the 
other.  The  vertical  pupil  of  the  eye  is  an 
exceptional  character  among  the  Bufonidae. 

TABLE   OF   CONVERGENCE   IN   BUFONID^. 
MYOBATRACHUS.  RHINOPHRYNUS. 

1.  Distribution.     Australia.  Mexico. 

2.  Shoulder  girdle.      Epicor-          The    same,    except    sternum 
acoid    cartilages    narrow    and      rudimentary. 

scarcely  overlapping;  no  omo- 
sternum ;  sternum  (or  meta- 
sternum)  cartilaginous,  ossified 
or  calcified  on  the  median  line, 

3.  Eye.     Pupil  vertical.  Pupil  vertical. 

4.  Ear.     Tympanum  distinct.          Tympanum  absent. 

5.  Tongue    small,    elliptical,          Tongue    elongate,    subtrian- 
entire  and  free  behind.  gular,  free  in  front. 

6.  Fingers  and  toes  free.  Fingers  free,  toes  webbed. 

7.  Limbs  very  short,  adapted          Limbs  very  short, 
for  burrowing. 

8.  Male  with  a  subgular  vocal          Male  with  two  lateral  vocal 
sac.  sacs  internal  behind  the  angle 

of  the  mouth. 

9.  Habits.      Feeding  on  ter-          Feeding  on  termites  and  ants, 
mites  and  ants. 

1  H.   Gadow,    "Amphibia    and    Reptiles,"   Camb.  Nat.    Hist., 
1901,  pp.  166  and  227. 

2  G.  A.  Boulenger,  Catalogue  Batr.  Sal,  1882,  pp.  328-329. 


PLACENTATION  103 

The  phenomena  of  placentation,  according  to 
Hubrecht,  are  "  intimately  related  to  the  higher 
development  which  characterises  the  mammalia 
as  against  the  lower  Vertebrates."  This  means 
that  the  intensive  nutrition  of  the  embryo  is  a 
prior  condition  to  the  development  of  brain 
power,  and  it  should  be  kept  in  mind  as  a 
counterpoise  to  Gaskell's  opinion  that  the  brain 
power  is  the  primary  directive  influence  in  evolu- 
tion. We  may  indeed  state  the  case  outright 
by  saying  that  cerebral  concentration  is  the  end 
and  not  the  beginning  of  evolution. 

The  history  of  the  mammalian  placenta  affords 
many  examples  of  convergence.  Diffuse  placenta- 
tion, in  which  the  surface  of  the  maternal  uterine 
mucous  membrane  is  thrown  into  a  dense  net- 
work of  folds  and  crypts  into  which  correspond- 
ing folds  or  villi,  diffused  over  the  entire  surface 
of  the  blastocyst,  fit  without  any  strong  adhesion 
to  the  uterine  wall,  has  usually  been  regarded  as 
a  primitive  condition  ;  and  the  arrangement  seen 
in  the  horse  and  the  pig  has  always  been  looked 
upon  as  the  prototype  of  the  diffuse  placenta. 
Hubrecht,  however,  gives  weighty  reasons  for 
thinking  that  it  is  due  to  a  secondarily  simplified 
process,  or  descent  with  simplification,  in  the 
course  of  which  the  intense  phagocytic  activity 
of  the  trophoblast  or  outer  foetal  envelope  had 
subsided  and  had  given  place  to  a  diffused  osmotic 


104  SPECIAL   CONVERGENCE 

activity  usually  accompanied  by  great  increase 
in  the  size  of  the  blastocyst. 

Similar  simplification  and  change  of  function 
leading  to  parallel  results  have  occurred  in  other 
orders  of  mammals,  e.g.,  in  certain  Edentates 
(Manis  the  pangolin,  or  scaly  ant-eater)  and  in 
the  Lemurs.  Hubrecht  expresses  the  hope  that 
his  discussion  of  the  facts  will  prevent  any  future 
attempt  to  place  Ungulates  and  Lemurs  on  the 
same  level  of  so-called  primitive  placentation. 

In  the  same  way  the  old  hypothesis  which 
maintained  that  the  early  villiferous  state  of  the 
human  blastocyst,  in  the  phase  called  "  Reichert's 
ovum,"  recapitulates  ontogenetically  a  diffuse 
phase  to  which  the  discoid  stage  succeeded 
later,  ought  to  be  discarded,  because,  inter  alia, 
Reichert's  ovum  is  enclosed  by  a  decidua  reflexa 
sen  capsularis,  and  is  not  freely  suspended  in  the 
uterine  cavity  as  are  the  blastocysts  which  show 
diffuse  placentation.  Furthermore,  Hubrecht 
adds  that  this  "phenomenon  of  encapsulisation 
inside  the  mucosa  has  appeared  independently 
in  more  than  one  order  of  mammals."  The 
arrangement  in  question,  by  which  the  develop- 
ing blastocyst  is  withdrawn  from  the  uterine 
lumen  and  enclosed  by  a  decidua  capsularis,  has 
been  realised  in  man  and  anthropoid  apes,  in 
different  genera  of  rodents,  in  the  spiny  hedge- 
hog Erinaceus,  in  the  spineless  Malayan  hedge- 


PLACENTATION  105 

hog  Gymnura,  and  rather  less  completely  in  the 
Chiroptera  (bats). 

Similarly  the  discoidal  form  of  the  placenta 
gives  no  indication  of  affinity  between  the  widely 
different  mammals  which  present  it.  It  is,  as 
Hubrecht  says,  a  temporary  production,  the  dis- 
coidal shape  being  of  no  value  when  considering 
questions  of  affinity.  He  instances  the  discoid 
placenta  of  the  mole  out  of  which  the  allantoic 
villi  are  withdrawn  at  birth  like  the  fingers 
out  of  a  glove,  the  young  when  born  being 
"  enveloped  in  the  allantois  with  the  fully  extracted 
villi  forming  a  woolly  covering  to  that  foetal  in- 
volucrum"  ;  the  discoid  placenta  of  Galeopithecus, 
containing  lacunae  filled  with  maternal  blood  and 
imbedded  in  the  uterine  wall ;  "  the  discoid 
placenta  of  the  rabbit  and  of  Tarsius  which, 
when  full  grown,  is  attached  to  the  mother  by  a 
stalk  of  much  smaller  diameter  than  the  placenta 
itself;  the  discoid  placenta  of  the  hedgehog  and 
of  man,  the  latter  with  its  loose  and  floating 
villi  as  against  the  dense  trellis -work  of  villi 
and  trophoblast  in  the  former."1 

Before  leaving  the  subject  of  the  nutrition  of 
the  young  of  vertebrates,  one  more  matter  may 
receive  attention,  namely,  the  secretion  of  milk 
that  takes  place  in  the  mammary  glands  from 
which  the  mammalia  take  their  name.  According 

1  A.  A.  W.  Hubrecht,  "Early  Ontogenetic  Phenomena  in 
Mammals,"  Quart.  Journ.  Micr.  St.,  vol.  liii.,  1908,  see  pp.  70,  113, 
129,  142,  etc. 


io6  SPECIAL   CONVERGENCE 

to  Owen  (op.  cit .,  vol.  ii.  p.  160),  John  Hunter 
"made  many  interesting  observations  on  the  crop 
of  pigeons,  which  takes  on  a  secreting  function 
during  the  breeding  season,  for  the  purpose  of 
supplying  the  young  pigeons  in  the  callow  state 
with  a  diet  suitable  to  their  tender  condition. 
An  abundant  secretion  of  a  milky  fluid  of  an 
ash-grey  colour,  which  coagulates  with  acids  and 
forms  curd,  is  poured  out  into  the  crop  and  mixed 
with  the  macerating  grains.  This  phenomenon 
is  the  nearest  approach  in  the  class  of  birds  to 
the  characteristic  mammary  function  of  a  higher 
class ;  and  the  analogy  of  the  *  pigeon's  milk ' 
to  the  lacteal  secretion  of  the  mammalia  has 
not  escaped  popular  notice.  .  .  .  The  secretion 
consists  of  proteine  with  oil,  but  contains  no 
sugar  of  milk  nor  fluid  caseine." 

An  analogous  secretion  has  been  discovered 
by  Alcock  to  take  place  in  the  uterus  of  vivi- 
parous rays.  In  pregnant  females  of  sting-rays 
(Trygon\  of  the  eagle-ray  (Myliobatis\  and  in 
the  bat-ray  (Pteroplataa),  Alcock  found  that  the 
young  are  nourished  before  birth  by  a  milky 
secretion  that  exudes  from  glandular  filaments 
or  villi  on  the  inner  surface  of  the  uterine  wall, 
a  large  bundle  of  the  filaments  passing  through 
each  spiracle  into  the  pharynx  of  the  immature 
fish.  The  milk-secreting  filaments  are  penetrated 
by  a  capillary  network  in  the  meshes  of  which 
the  milk-glands  are  imbedded.  Each  filament 


SECRETION  OF   MILK  107 

is  provided  with  superficial  muscles  whose  con- 
traction must  serve  to  squeeze  the  milk  out. 
These  vascular  villi  give  a  shaggy  appearance 
to  the  mucous  membrane  of  the  oviduct,  and 
Alcock  describes  them  as  " dripping  with  milk" 
and  discharging  a  creamy,  albuminous  fluid.1  This 
method  of  intra-uterine  nutrition  of  the  young  by 
a  special  secretion  of  the  uterine  glands  offers  at 
once  a  great  contrast  and  a  remarkable  parallel 
to  the  corresponding  phenomena  in  mammals. 

Every  system  of  organs  throughout  the  animal 
kingdom  will  be  found  to  yield  abundant  instances 
of  convergence.  An  excellent  example  of  what 
may  be  called  gastral  convergence  is  exhibited 
in  the  structure  of  the  gizzard  of  some  Teleostean 
fishes.  In  the  first  place,  as  between  the  pyloric 
gizzard  of  the  few  fishes  which  possess  one  and 
the  pyloric  gizzard  which  is  so  characteristic  of 
birds,  it  is  to  be  noted  that  the  most  absolute 
comparability  prevails,  differences  in  detail  not- 
withstanding. They  are  homoplastic  modifica- 
tions of  a  homologous  structure,  namely,  the 
pyloric  division  of  the  vertebrate  stomach.  The 
exceptional  occurrence  of  a  gizzard  in  fishes 2  and 
the  regular  presence  of  one  in  birds  give  no  more 
indications  of  affinity  than  does  the  gizzard  of 

1  A.   Alcock,   "A   Naturalist  in   Indian  Seas,"   London,    1902, 
see  pp.  71,  159,  210.     This  valuable  book  contains  a  bibliography 
of  the  published  work  of  the    Indian  Marine  Survey  ship  In- 
vestigator. 

2  Compare  also  the  gizzard  of  the  toothless  Ant-eaters. 


io8  SPECIAL   CONVERGENCE 

the  earthworm  or  that  of  some  molluscs.  The 
latter  are  examples  of  phyletic  convergence  not 
based  directly  on  special  homology ;  the  gizzard 
in  fishes  and  birds  is  a  case  of  class  convergence 
based,  as  we  have  seen,  upon  a  homogenetic 
foundation.  We  may  thus  once  more  distinguish 
clearly  between  convergence  with  homology  and 
that  without  it.  This  may  seem  rather  para- 
doxical, but  it  is  true  and  sufficiently  significant 
to  bear  repetition  :  the  pyloric  division  of  the 
stomach  is  homologous  in  fishes  and  birds ;  but 
the  gizzard  of  fishes,  which  is  a  direct  modifica- 
tion of  the  pylorus,  is  not  homologous  with  the 
gizzard  of  birds,  which  is  likewise  a  direct,  though 
independent,  modification  of  the  pylorus. 

In  that  rich  storehouse  of  facts,  Owen's 
"Anatomy  of  Vertebrates"  (1866),  the  gizzard 
of  the  Grey  Mullet  (Mugil)  is  thus  described 
(vol.  i.  p.  418) :  "The  cardiac  portion  here  forms 
a  long  cul-de-sac ;  the  pyloric  part  is  continued 
from  the  cardiac  end  of  this  at  right  angles  and 
is  of  a  conical  figure  externally ;  but  the  cavity 
within  is  reduced  almost  to  a  linear  fissure  by 
the  great  development  of  the  muscular  parietes, 
which  are  an  inch  thick  at  the  base  of  the  cone  ; 
and  this  part  is  lined  by  a  thick  horny  epithelium." 
In  other  cases  the  pylorus  will  be  found  to  be 
hard  and  muscular,  as  in  the  Red  Mullet  ( Upene- 
oides\  In  the  Clupeoid  genus  Megalops,  the 
pylorus  is  a  long,  straight,  porrect  ascending  tube, 


GIZZARD   IN   FISHES 


109 


rather  hard  at  its  distal  extremity  ;  in  another 
Clupeoid  genus  Dussumieria,  the  pylorus  appears 
as  an  arched  muscular  tube  lying  across  the  body 
cavity  from  left  to  right,  the  concavity  of  the  arch 
being  directed  forwards  (Fig.  9). 
In  a  species  of  Grey  Mullet 
from  Ceylon  which  I  identified 
as  Mugil  olivaceus  Day,  I  found 
the  gastric  ccecum  rudimentary 
and  the  cardiac  division  of  the 
stomach  was  followed  by  a  round, 
white,  muscular,  bulbous  gizzard  ; 
the  intestine  was  full  of  fine  sand  ; 
six  pyloric  cceca  embraced  the 
gizzard;  another  specimen  had 
only  five  pyloric  cceca.  What, 
however,  surprised  me  very 
much  at  the  time — and  I  have 
not  found  it  mentioned  in  any 
work  which  I  have  consulted 
since — was  the  discovery  of  an 
almost  precisely  similar  gizzard 
in  a  Clupeoid  fish,  Chatoessus 
nasus,  which  frequents  the  same  g-c-  Gastric  [cardiac] 

ccecum. 

back-waters  as  the  grey  mullet.  p,c.  Pyloric  cceca. 
In    this    species    (C.    nasus}   the      *  Destine, 
stomach  is  frankly  destitute  of  a  cardiac  ccecum, 
and  terminates  in  a  round,  muscular,  red,  bulbous 
gizzard.     In  one  of  the  specimens  dissected,  the 
gizzard  and  intestine  were  full  of  fine  sand  as  in 


FIG.  9. 

Intestinal  tract  of 
Dussutnieria. 
p.  Pylorus. 


no  SPECIAL  CONVERGENCE 

the  mullet ;  and  the  presence  of  sand  was  noted 
in  the  pyloric  cceca.  The  latter  arise  in  tufts 
along  the  length  of  the  duodenum  (Fig.  10). 


FIG.  10.  Dissection  of  the  alimentary  tract  of  Chatoessus  nasus 
from  the  left  side.  The  dotted  line  indicates  the  cut  edge 
of  the  body- wall. 

1.  Gall-bladder. 

2.  Visceral  nerve  lying  upon  the  cardiac  tube  of  the  stomach. 

3.  Air-bladder. 

4.  Ductus  pneumaticus. 

5.  Pyloric  gizzard  with  a  tuft  of  cceca  projecting  in  front  of  it. 

6.  Duodenum  (green-tinted  when  fresh). 

7.  Rectum. 

8.  Hind-portion  of  left  testis. 

9.  Vas  deferens. 

Here,  then,  we  have  two  fishes  belonging  to 
widely  separated  families,  though  pursuing  similar 
habits,  and  presenting  independently  an  identical 
modification  of  the  pyloric  division  of  the  stomach. 
I  confess  that  at  first  acquaintance  with  this  case 
I  began  to  distrust  my  own  eyes  ;  perhaps  it  will 
not  strike  the  reader  of  these  pages  so  forcibly  ; 
but  I  submit  it  as  one  of  the  most  stringent 
object  lessons  in  homoplasy  imaginable. 

It  may  be  admitted  that  wherever  there  is  a 


GIZZARD   IN   FISHES  in 

distinct  pyloric  division  of  the  stomach,  the 
material  for  a  hard  muscular  gizzard  exists.  It 
is  obvious  that  the  constitution  of  the  alimen- 
tary canal  depends  upon  the  nature  of  the  food. 
Comparative  anatomy  and  physiology  teach  us 
that  the  digestive  tract  reacts  to  different  condi- 
tions of  nutrition;  but  not  many  such  unequivocal 
demonstrations  of  the  independent  acquisition, 
within  the  limits  of  an  order,  of  a  compact 
specialised  structure  as  that  described  above, 
can  be  pointed  to. 

Regarding  the  families  of  the  Mugilidae  and 
Clupeidae  in  their  entirety,  we  have  now  con- 
sidered two  characters  with  respect  to  which 
they  converge,  namely,  the  adipose  eyelids  (above, 
p.  94)  and  the  pyloric  gizzard.  In  both  of  these 
cases,  as  well  as  in  that  of  the  pectoral  fins  of 
the  flying  fishes,  we  have  anatomically  identical 
structures  arising  independently  from  a  common 
origin.  Facts  of  this  nature  apparently  take  the 
ground  away  from  any  intelligible  conception  of 
homology — but  only  apparently.  The  relations 
can  be  illustrated  graphically,  as  shown  in  the 
diagram  (Fig.  n). 

The  gizzard  of  the  mullet  was  known  to  John 
Hunter,1  who  tells  us  that  of  all  the  fish  seen 

1  John  Hunter,  "  Observations  on  the  Gillaroo  Trout,  commonly 
called  in  Ireland  the  Gizzard  Trout,"  Phil.  Trans.)  1774;  reprinted 
in  his  "Animal  Oeconomy,"  which  was  republished  in  vol.  iv.  of 
Hunter's  Works,  edited  by  James  F.  Palmer,  1835-1837.  Here 
also  is  to  be  found  Hunter's  paper,  "  On  a  Secretion  in  the  Crop 
of  Breeding  Pigeons,  for  the  Nourishment  of  their  Young." 


ii2  SPECIAL  CONVERGENCE 

by  him  the  mullet  offered  the  most  complete 
instance  of  this  structure,  its  strong  muscular 
stomach  being  evidently  adapted,  like  the  gizzard 
of  birds,  to  the  two  offices  of  mastication  and 
digestion.  The  stomach  of  the  gillaroo  trout,  in 


FIG.  ii.     Parallelism  with  convergence  between  two  families 
of  fishes,  Mugilidas  and  Clupeidas. 

Hunter's  experience,  held  the  second  place.  But 
the  gizzard  of  Chatoessus,  described  above,  holds 
a  position  of  equality  with  that  of  the  mullet. 

It  should  be  added  that  Darwin  ("Origin  of 
Species,"  p.  235)  referred  to  the  luminous  organs 
of  insects  of  distinct  families  and  diverse  topo- 
graphy as  a  phenomenon  parallel  with  that  of 
the  electric  organs  of  fishes. 


CHAPTER   VIII 

HABITUDES  AND  ATTITUDES 
(BIONOMICAL  CONVERGENCE) 

IDENTITY  of  habitat  occasions  convergence  of 
habits  on  the  part  of  diverse  animals.  A  great 
deal  of  this  has  been  implied  in  the  preceding 
chapters,  but  more  remains  to  be  said.  The  three 
principal  functions  of  animal  life,  at  least  above 
the  level  of  the  sponge,  are  metabolism,1  repro- 
duction, and  neuration,  the  last  being  at  once  the 
master  and  the  servant  of  the  others,  and  leading 
on  from  a  lower  to  a  more  advanced  cerebration. 
The  number  of  phyla  or  leading  types  of  the 
animal  kingdom  is  considerable,  and  there  are 
only  these  three  primary  functions  upon  which 
to  ring  the  changes. 

If  there  is  one  aspect  of  convergence  more 
widespread  than  another,  it  is  that  of  cerebral 
convergence,  which  is  associated  with  the  pheno- 
menon of  cephalisation  or  head-formation ;  and 
we  can  infer  from  this  circumstance  how  hope- 
less must  be  the  search  for  neural  homology  as 

1  Metabolism  comprises  nutrition,  respiration,  and  excretion. 
113  H 


ii4  HABITUDES   AND  ATTITUDES 

between  parallel  types  of  the  animal  kingdom. 
The  cerebration  of  the  ant  is  comparable,  in 
several  of  its  manifestations,  with  that  of  the 
higher  mammals,  but  its  evolution  is  distinct 
and  its  mechanism  different.  The  comparison 
between  the  two  forms  is  most  profitable  in  the 
abstract ;  but  it  points  away  from  homology,  not 
towards  it.  Perhaps  either  direction  leads  round 
in  a  vicious  circle  to  the  same  goal,  namely,  the 
point  from  which  we  started  ;  but  we  may  gather 
much  by  the  way  if  we  do  not  stray  too  far  from 
the  track  and  lose  ourselves  in  a  jungle  of  facts 
and  speculations. 

National  life  is  chiefly  controlled  by  the  desire 
to  capture  markets.  Animal  life  is  chiefly  con- 
cerned with  the  occupation  of  feeding  grounds. 
In  general,  any  given  feeding  area  can  support 
a  very  mixed  population,  and  the  association  of 
forms  which  batten  upon  any  particular  source 
of  supply  constitutes  what  has  been  termed  the 
bioccenosis  [Dahl]  of  that  centre.  This  com- 
munity of  interests  is  a  phase  of  symbiosis  or 
commensalism,  not  based  upon  mutual  advantage 
nor  even  upon  mutual  toleration,  in  the  first 
instance,  but  upon  strict  independence  and  self- 
help. 

All  the  species  which  take  part  in  such  associa- 
tions may  be  said  to  converge  towards  a  common 
centre  of  sustenance,  and  it  is  generally,  though 
by  no  means  invariably  noted  that  the  assemblage 


ECTOPARASITES  115 

is  a  heterogeneous  one,  the  species  composing  it 
belonging  to  so    many  genera,  and  the   genera 
usually  belonging  to  different  families  or  orders. 
How   far    this   community   of    habitat   leads   to 
structural  convergence  is  not  clear,  because  the 
anatomical    characters   of  the    associated   forms 
have   not   been  worked  out    in   sufficient   detail 
from  this  point  of  view.     A  case  in  point  is  the 
overlapping   of    head    and    pronotum    in    some 
ectoparasitic  insects  to  which  attention  has  been 
drawn  recently  by  Dr  K.  Jordan.1     In  Arixenia, 
an  apterous  earwig  found  in  the  nursing  pouch  of 
the  naked  bat  of  the  Sunda  Islands,  the  occipital 
margin  of  the  head  is  slightly  concave,  without 
a  sharp  edge,  and  is  not  closely  applied  to  the 
pronotum.     In  Hemimerus,  another  apterous  ear- 
wig  bearing   a   superficial  resemblance  to  some 
Blattidae,  from  the  African  murine  genus  Crice- 
tomys,  the  hind  edge  of  the  head  projects  back- 
wards,   overlapping    the    pronotum    to    a   slight 
extent.     Dr  Jordan  adds  that  this  overlapping, 
which    is    exceptional    among    insects,    is    best 
known  in  fleas  and  in  some  Hemiptera  parasitic 
on   bats.      In    the   beaver   parasite,  Platypsyllus 
castoris,  the  head  and  pronotum  fit  well  together, 
and  there  is  a  comb  of  spines  extending  from  the 
edge  of  the  head  on  to  the  thorax,  bridging  over 
the  gap  which  might  be  formed  when  the  head 

1   K.    Jordan,    "  New  Apterous  Earwig  (Arixenia)J    Novitat. 
y  xvi.,  1909,  p.  318. 


u6  HABITUDES  AND  ATTITUDES 

is  bent  down.  The  overlapping  between  head 
and  pronotum  of  parasites  which  live  in  the  fur 
of  mammals  renders  the  surface  uniform  and 
more  suitable  for  gliding  through  the  fur,  and 
is  a  secondary  development  which  has  taken 
place  independently  in  these  not  nearly  related 
insects  [Jordan,  loc.  cit.~\. 

The  identification  of  the  species  of  arthropods, 
land-planarians,  earthworms,  and  molluscs  found 
in  or  about  a  single  fallen  log  is  a  matter  of 
almost  unspeakable  difficulty  under  the  present 
conditions  of  zoological  nomenclature  and  publica- 
tion. By  far  the  greatest  amount  of  biological 
interest  connected  with  a  particular  species  may 
be  due  to  the  association  under  which  it  was 
found,  but  this  is  a  matter  of  the  least  possible 
systematic  importance,  except  in  the  case  of 
parasites.  The  mere  commingling  of  different 
forms,  apart  from  structural  details,  constitutes 
a  phase  of  our  subject  which  is  worthy  of  notice, 
and  may  be  termed  bioccenotic  convergence.1 

It  is  known  and  will  be  found  recorded  in 
Darwin's  "Monograph  of  the  Cirripedia,"  that 
barnacles  are  frequently  found  attached  by  their 
peduncles  to  the  skin  of  sea-snakes  (Hydrophidse). 
Occasionally  a  considerable  cluster  of  barnacles 
may  be  fixed  upon  the  flattened  tail  of  a  slender 
snake,  as  in  the  case  of  a  Hydrus  platurus 

1  Such   expressions    as    this    are   introduced  merely  for  con- 
venience of  reference  and  classification  of  the  phenomena. 


ASSOCIATIONS  117 

from  the  coast  of  Ceylon,  now  preserved  in  the 
Colombo  Museum,  whose  tail  was  beset  with  a 
group  of  barnacles  composed  of  two  species, 
Lepas  anserifera  and  Cone  ho  derm  a  hunteri.  The 
barnacles  are  not  ectoparasitic,  since  they  do  not 
feed  upon  the  skin  of  the  snake  nor  do  they 
assist  the  snake  in  any  way ;  on  the  contrary, 
their  presence  must  have  seriously  impeded  its 
movements.  This  snake  is  sufficiently  protected 
from  larger  enemies  by  its  warning  coloration 
(black  and  yellow)  and  by  its  possession  of  poison 
fangs.  Moreover,  the  barnacles  thrive  equally 
well  when  attached  to  floating  bottles  and  drift- 
ing spars,  and  the  sea-snake  in  question  was 
merely  their  facultative  vehicle. 

The  relation  of  barnacles  to  the  skin  of  sea- 
snakes  is  somewhat  analogous  to  a  remarkable 
case  of  association  between  certain  Hydroid 
polyps  (Stylactis  minoi)  and  a  small  rock  perch, 
Minous  inermis,  which  has  been  found  in  several 
places  off  the  west  coast  of  India  at  depths  of 
45-150  fathoms.  The  skin  of  the  fish  is  beset 
with  the  commensal  polyps  which  have  never 
been  found  elsewhere,  and  Colonel  Alcock  (op. 
cit.,  1902)  thinks  that  they  help  to  conceal  the 
fish  from  its  enemies,  in  that  they  play  the 
same  part  which  is,  in  other  cases,  performed  by 
frond -like,  cutaneous  filaments.  In  both  cases 
feeding  is  carried  on  independently  of  the  verte- 
brate host.  The  barnacles,  named  above,  are 

H  2 


n8  HABITUDES  AND   ATTITUDES 

sedentary  animals  that  require  to  be  kept  in 
motion  at  the  surface  of  the  sea ;  while  the 
hydroid  is  a  sedentary  species  which  requires  an 
adventitious  mobility  at  the  bottom  of  the  sea. 

The  Tubificidae  are  a  family  of  small  fresh- 
water Annelid  worms,  allied  to  earthworms,  which 
live  in  the  mud  of  water  -  courses,  sometimes 
occurring  in  almost  pure  cultures  of  innumer- 
able individuals.  They  keep  the  head  and 
fore-body  buried  in  the  mud,  whilst  the  hinder 
portion  of  the  body,  through  which  respiration 
is  effected,  is  kept  constantly  waving  as  near 
the  surface  of  the  shallow  water  as  possible.1 
When  alarmed,  an  entire  colony  will  instantly 
withdraw  out  of  sight  into  the  mud  as  with  one 
consent.  Living  in  the  same  environment  and 
sometimes  in  company  with  the  worms  are  to 
be  found  the  larvae  of  midge  flies  (Chironomus]. 
They  have  the  same  habit  of  waving  the  body 
in  the  water  and  the  respiratory  processes  occur 
at  the  hinder  end  ;  and,  most  singular  coincidence 
of  all,  the  blood  of  the  insect  larvae  is  coloured 
red  with  haemoglobin  like  that  of  the  worms. 

In  his  description  of  the  larva  of  Chironomus, 
commonly  called  the  "  Blood-worm,"  Professor 
Miall  ("  Natural  History  of  Aquatic  Insects," 
London,  1895)  says:  "When  undisturbed,  they 

1  The  habit  of  waving  the  posterior  end  of  the  body  in  the  water 
has  been  noted  for  Tubifex  rivulorum  by  L.  Atheston,  Anat.  Anz^ 
xvi.,  p.  497  ;  I  have  observed  it  in  the  case  of  an  unnamed  species 
of  Limnodrilm  from  Ceylon,  as  described  in  the  text. 


SOCIAL  INSECTS  II9 

may  often  be  seen  to  push  the  head-end  well 
out  of  the  burrow  for  purposes  of  feeding ;  at 
other  times  the  tail  -  end  is  pushed  out  and 
waved  to  and  fro  in  the  water,  as  a  help  to 
respiration."  This  may  be  compared  with 
Atheston  on  Tubifex\  "It  forms  flexible  tubes 
open  at  both  ends,  of  particles  of  soil  cemented 
by  mucus.  When  contracted,  the  worm  lies 
wholly  within  the  tube.  When  active,  the 
posterior  end  (third  or  half  of  the  body)  pro- 
jects into  the  water,  waving  about  in  a  rapid, 
undulatory  manner,  while  from  the  other  end  of 
the  tube  is  protruded  the  anterior  end  of  the 
worm  which  is  thrust  about  through  the  mud 
in  search  of  food."  Limnodrilus,  as  observed 
by  me  in  Ceylon,  forms  dense  aggregates  of 
individuals  surrounded  by  mud,  but  does  not 
form  definite  tubes  which  can  be  isolated  from 
the  clumps. 

No  example  of  bionomical  convergence  is 
more  remarkable  than  that  which  is  presented 
by  the  social  insects,  ants,  bees,  wasps,  and 
termites.  The  termites  so  much  resemble  ants 
in  their  mode  of  life  and  social  organisation 
that  they  are  commonly  known  in  the  tropics 
as  white  ants,  though  they  are  not  ants  and 
are  not  always  white.  Not  only  have  they 
no  direct  genetic  relationship  with  the  true 
ants  (Formicidae,  order  Hymenoptera),  but  the 
latter  are  among  their  most  formidable  enemies. 


120  HABITUDES  AND   ATTITUDES 

A  minute  ant  will  overpower  a  termite  twice 
or  three  times  its  own  size,  seizing  it  from 
behind  round  the  middle,  causing  it  to  twist 
and  writhe,  but  never  loosening  its  hold  upon 
the  doomed  victim.  The  social  differentiation 
of  workers,  soldiers,  kings,  and  queens  is  the 
same  in  the  ant  family  and  in  the  termite 
family ;  but  the  feeding-habits  are  quite  differ- 
ent, and  termites  do  not  keep  slaves.  The 
nearer  relationships  of  the  termites  are  with 
cockroaches  and  earwigs.  Froggatt  thinks  that 
the  discovery  of  the  giant  termite  from  Port 
Darwin,  Mastotermes  darwiniensis,  "brings  them 
almost  into  touch  with  the  family  Blattidae."  l 

Although  termites  do  not  employ  slaves  as 
some  ants  do,  yet  they  entertain  guests  which 
are  known  as  termitophilous  insects,  as  dis- 
tinguished from  the  myrmecophilous  insects 
which  frequent  ants'  nests.  Interesting  exhibi- 
tions of  mimicry  and  other  special  adaptations 
result  from  these  associations.  The  phase  of 
mimicry  which  involves  the  assumption  of  the 
ant-facies  is  called  myrmecoidism  by  Wasmann. 
He  distinguishes  passive  mimicry,  or  the  decep- 
tive resemblance  of  the  outward  form,  from  active 
mimicry,  which  consists  in  an  imitation  of  the 
behaviour  of  the  hosts.  The  mimicry  which  has 

1  Walter  W.  Froggatt,  "White  Ants."  Department  of  Agriculture, 
New  South  Wales.  Miscellaneous  Publication  No.  874,  1905  ; 
containing  a  bibliography  of  papers  dealing  with  Australian 
termites. 


MYRMECOIDISM  121 

for  its  end  the  deception  of  the  hosts  is  further 
distinguished  from  other  forms  of  myrmecoidism 
amongst  Arthropoda ;  one  phase  of  the  latter 
is  merely  a  morphological  family  resemblance 
without  apparent  biological  significance. 

Some  guests  are  welcome,  others  are  hostile. 
An  example  of  the  latter  is  a  small  beetle, 
Myrmedonia,  some  species  of  which  are  myrme- 
cophilous,  whilst  others  are  termitophilous.  They 
lie  in  wait  in  obscure  corners  of  the  nest,  whence 
they  fall  upon  isolated  ants  and  tear  them  to 
pieces ;  on  the  other  hand,  they  have  much 
ado  on  their  part  to  escape  the  vigilance  of  the 
warrior  ants.  The  myrmecophilous  species  of 
Myrmedonia  resemble  the  ants  with  which  they 
associate  both  in  colour  and  in  form  ;  so  much 
so  that  Wasmann  himself  has  been  repeatedly 
deceived  at  a  first  glance.  The  object  of  this 
mimicry  is  demonstrated  by  Wasmann  to  be 
the  optical  delusion  of  the  ants. 

Among  the  numerous  Oriental  species  of 
termitophilous  Myrmedonia  there  is  no  such 
mimicry,  although  the  Termes  soldiers  are  as 
capable  of  defence  as  are  the  ant  soldiers,  but 
with  this  difference,  that  they  are  completely 
blind.  In  the  nests  of  the  termite  genus  Hodo- 
termes,  whose  soldiers  possess  well  -  developed, 
facetted  eyes,  no  Myrmedonia  has  been  found.1 

1  E.  Wasmann,  "Die  psychischen  Fahigkeiten  der  Ameisen," 
second  edition.    Published  in  Zoologica^  Bd.  xi.,  Heft  26.    Stuttgart, 


122  HABITUDES   AND   ATTITUDES 

Another  phase  of  bionomical  convergence  is  that 
contained  in  the  phenomena  of  direct  and  indirect 
development  amongst  many  invertebrate  animals. 
Direct  development  is  straightforward,  gradual, 
without  striking  metamorphosis,  and  must  be 
regarded,  in  a  certain  sense,  as  normal.  Works 
which  deal  with  direct  development  are  apt  to 
become  standard  and  classical ;  they  give  the 
clue  to  fundamental  homologies  and  generalisa- 
tions. The  theory  of  the  coelom  could  hardly 
have  been  established  upon  an  indirect  course 
of  development ;  and  on  the  other  hand,  the 
relationship  between  the  Enteropneusta  and 
Echinodermata  would  not  have  been  recognised 
so  easily  from  the  direct  development,  although 
the  larva  of  Asterina  gibbosa  would  doubtless 
have  called  for  comparison  with  Bateson's  larva 
of  Balanoglossus.1  The  differences  between  the 
larvae  of  certain  marine  organisms  consist  in  the 
littoral  or  stereotropic  habit  of  the  direct  forms 
as  contrasted  with  the  pelagic  or  free-swimming 
(pleotropic)  habit  of  the  indirect  forms ;  and  they 
are  anticipated  by  the  size  of  the  egg  and  the 
amount  of  food  yolk  it  contains.  A  very  small 
egg  will  develop  into  a  pelagic  larva ;  a  relatively 
large  egg  will  develop  directly  into  a  larva  in 

1909,  pp.  190,  5  plates.  Contains  bibliography  of  Wasmann's 
numerous  contributions  to  the  knowledge  of  myrmecophily  and 
termitophily. 

1 1  employ  this  name  in  the  old  sense,  just  as  Amphioxus  is  still 
extensively  used,  without  trenching  upon  questions  of  nomenclature. 


TORN  ARIA  123 

which  the  form  of  the  adult  can  be  more  or  less 
clearly  discerned,  unless,  as  in  the  case  of  the 
above-named  starfish,  the  adult  form  has  suffered 
a  peculiar  change  from  a  bilaterally  symmetrical 
type. 

The  general  name  of  the  pelagic  larval  form 
of  those  Enteropneusta  (Balanoglossida),  which 
produce  small  ova  not  exceeding  0.15  mm.  in 
diameter,  is  Tornaria.  All  members  of  the 
families  Ptychoderidae  and  Glandicipitidae  (or 
Spengelidse)  produce  such  eggs,  The  Harri- 
manidae  produce  large  yolky  eggs  from  0.4  mm. 
in  Dolichoglossus  kowalevskii  to  as  much  as 
1.5  mm.  in  Stereobalanus  kupfferi.  Tornaria 
was  discovered  in  1848  by  the  famous  German 
physiologist  Johannes  Miiller  of  Berlin,  who 
thought  that  it  was  an  Echinoderm  larva ;  its 
real  nature  was  determined  in  1869  by  Professor 
Elias  Metschnikoff. 

It  is  well  known  how  much  the  quantity  of 
food-yolk  in  the  egg-cell  affects  the  course  not 
only  of  the  segmentation-stages,  but  also  of  the 
subsequent  embryonic  and  larval  development; 
and  that,  under  normal  conditions  of  nutrition, 
the  size  of  the  egg  is  a  fairly  constant  specific 
character. 

The  dimensions  of  the  eggs  of  different  animals 
do  not  always  vary  in  proportion  to  the  bulk  of 
the  progenitors.  Amongst  existing  birds,  where 
oviparity  is  the  rule  without  any  exceptions,  the 


124  HABITUDES   AND   ATTITUDES 

smallest  species  yield  the  smallest  eggs  and  the 
largest  birds  lay  the  largest  eggs,  but  the  inter- 
vening sizes  show  great  fluctuations,  as  may  be 
seen  by  comparing  the  egg  of  a  Megapode  with 
that  of  a  Jungle  Fowl. 

Amongst  reptiles  there  are  many  exceptions 
to  the  rule  of  oviparity,  but  there  is  no  known 
correlation  between  lecithality,  or  the  relative 
quantity  of  yolk  in  the  egg,  and  viviparity ;  this 
latter  habit  not  leading  to  a  reduction  of  the 
vitellus,  although  it  may  affect  the  shell-formation, 
causing  the  complete  absence  of  calcareous  de- 
posits, so  that  the  embryo  shows  clearly  through 
its  thin  transparent  envelopes.1  The  intra-uterine 
development  of  a  yolk  -  laden  egg  is  a  special 
phase  of  incubation  or  brood  -  nursing,  and  has 
been  assumed  independently  by  some  arboreal 
and  deserticolous  lizards  of  diverse  families  in 
distant  parts  of  the  world. 

A  fundamental  rule  with  regard  to  methods 
of  propagation  is  that  oviposition  preceded  vivi- 
parity, but  this  rule  does  not  assist  in  determining 
whether  the  ancestors  of  a  particular  class  were 
egg-layers  or  vivipars,  and  consequently  whether 
the  egg-laying  members  of  a  group  where  vivi- 
parity predominates  are  primitive  in  that  respect. 
Lecithality,  oviparity,  and  viviparity  are  or  may 
be  independent  phenomena.  It  is  probable,  for 

1  Cf.  A.  Willey,  1906,  "Viviparity  of  Cophotis  ceylanica  and 
Oviparity  of  Ceratophora  stoddartii"  Spolia  Zeylanica^  vol.  iii., 
pp.  235-237  and  figure. 


PERIPATUS  125 

example,  that  the  egg -laying  habit  of  the 
Australian  monotremes  is  due  to  early  regression 
or  descent  with  simplification  in  a  primitive  type, 
while  the  lecithality  of  their  eggs  is  not  a  palin- 
genetic  or  high  ancestral  quality,  but  adaptive 
or  cenogenetic.1  The  abundance  and  scarcity 
of  yolk  may  be  alike  primitive  and  secondary 
features  within  certain  defined  limits,  in  a  manner 
analogous  with  the  plurality  and  duality  of  teats 
in  mammals.2 

It  is  not  only  among  mammals  that  we  find 
large  eggs  associated  with  oviparity  and  small 
eggs  with  viviparity  and  placentation  ;  and  not 
only  among  marine  organisms  do  we  find  large 
eggs  associated  with  direct  development  and 
small  eggs  with  indirect  development. 

Peripatus  is  a  soft-bodied,  cryptozoic  animal, 
living  in  the  tropics  under  logs  and  leaves, 
whose  body  has  the  consistency  of  a  caterpillar, 
and  whose  feet  are  numerous  like  those  of  a 
centipede  but  not  jointed.  It  is  known  by  no 
other  name  except  that  of  its  own  division  of 
the  Appendiculata,  namely,  the  Onychophora. 
The  class  name  Prototracheata  was  applied  by 
Moseley  to  this  group,  but  since  that  time  it 
has  been  recognised  that  tracheate  animals  do 
not  form  a  homogeneous  series,  as  will  be  ex- 
plained more  fully  below. 

1  This  is  Professor  Hubrecht's  view  (/.£•.,  1908). 

2  Cf.  A.  Willey,  "The  Lacteal  Tract  of  Loris  gratilis?  Spolia 
Zeylanica,  vol.  iii.,  1905  (1906),  pp.  160-162  and  figure. 


is6  HABITUDES  AND   ATTITUDES 

The  Onychophora  furnish  instructive  gradations 
from  placentation,  at  one  extreme,  to  oviposition 
at  the  other,  the  sizes  of  the  eggs  increasing  part 
passu  from  minimal  to  maximal  dimensions.  The 
species  show  a  peculiar  correlation  between  their 
geographical  distribution  and  their  morphological 
differentiation  ; l  and  to  some  extent,  though  not 
entirely,  the  modes  of  development  vary  with 
the  distribution.  The  placental  condition  of  the 
small  -  egged  South  American  species  is  highly 
specialised  ;  the  oviparous  habit  of  some  Austra- 
lasian species  2  is,  to  my  mind,  clearly  secondary. 
As  for  the  species  themselves,  one  form  is  not 
appreciably  more  primitive  than  another. 

In  this  group,  therefore,  the  two  extreme 
methods  of  nutrition  of  the  embryo  are  equally 
cenogenetic.  There  still  remain  three  inter- 
mediate states  of  the  embryo,  namely,  the  tropho- 
blastic  development  of  the  small  -  egged  New 
Britain  species,  and  also  of  a  South  African 
species ;  the  normal  or  direct  development  of 
the  Cape  species ;  and  the  yolky  or  meroblastic 
development  of  some  Australian  and  Malayan 
species. 

Just  as  among  aquatic  organisms  which  dis- 
charge their  spawn  freely  into  the  water,  a 

1  Cf.  Adam  Sedgwick,  "The  Distribution  and  Classification  of 
the   Onychophora,"   Quart.  Journ.   Micr.  Sc.,  vol.  lii.,   1908,  pp. 
379-406. 

2  Cf.  A.  Dendy,  "  On  the  Oviparous  Species  of  Onychophora,5' 
Quart.  Journ.  Micr.  Sc.>  vol.  xlv.,  1902,  pp.  363-414. 


OPTIMUM   CONDITIONS  127 

distinction  has  to  be  made  between  direct  and 
indirect  development,  so  in  the  intra  -  uterine 
development  of  Peripatus  a  similar  contrast 
occurs.  What  is  meant  by  a  normal  course  of 
development  is  one  that  is  not  disturbed  by 
adventitious  factors.  We  must  assume  certain 
optimum  conditions  which  need  by  no  means 
be  the  most  primitive  and  indeed  on  a  priori 
considerations  are  not  likely  to  have  been  the 
earliest  conditions  encountered  by  the  amphibious 
palaeozoic  prototracheate  ancestors  of  our  terres- 
trial arthropods.  Optimum  conditions  may  be 
brought  about  by  the  acquisition  of  a  little  yolk 
in  the  egg  or  by  the  loss  of  a  little  yolk  ;  and 
this  may  make  a  considerable  difference  in  the 
behaviour  of  the  embryos  or  larvae,  as  the  case 
may  be. 

All  methods  of  development  may  be  regarded 
as  more  or  less  cenogenetic,  and  the  selection  of 
a  particular  type  as  primitive  may  be  purely 
arbitrary ;  but  there  can  hardly  be  two  opinions 
as  to  a  normal  or  direct  course  which  can  be 
established  as  a  legitimate  standard  or  constant. 
Any  life-history  can  recapitulate  certain  ancestral 
or  palingenetic  features,  some  more  recent,  others 
more  remote.  Different  species  may  provide 
seemingly  identical  conditions  for  the  maturation 
of  the  ova  and  the  development  of  the  young ; 
but  it  does  not  follow  that  the  reactions  or  adap- 
tations to  these  conditions  will  be  identical. 


u8  HABITUDES   AND   ATTITUDES 

When  both  conditions  and  reactions  in  different 
specific  groups  are  identical,  convergence  based  on 
homology  ensues.  Thus  the  trophoblastic  vesicle 
of  Peripatus  novcz-britannice  is  doubtless  homo- 
logous with  that  of  the  South  African  P.  sedg- 
wickii\  but  the  adaptation  which  produced  the 
structure  out  of  the  primitive  matrix  may  have 
been  evolved  independently,  i.e.,  along  closely 
parallel  lines  in  the  two  species.  If  this  is  so  it 
is  an  example  of  close  specific  convergence. 

In  accordance  with  the  Trophoblast  Theory,  the 
trophoblast,  wherever  it  occurs,  is  to  be  defined 
as  an  outer  larval,  or  extra-embryonic  envelope 
or  appendage  which  has  acquired  phagocytic 
properties  in  order  to  provide  for  the  intra-uterine 
nutrition  of  viviparous,  terrestrial  animals  which 
have  descended  from  oviparous,  aquatic  ancestors. 
In  accordance  with  the  theory  of  Larval  Forms 
amongst  aquatic  organisms  other  than  Arthro- 
poda,  what  may  be  termed  by  contrast  the 
kinetoblast  or  outer  ciliated  investment  of  the 
larva  has  acquired  special  locomotor  properties, 
frequently  concentrated  along  definite  tracts,  in 
order  to  provide  for  the  pelagic  life  and  inde- 
pendent nutrition  of  the  young. 

From  whatever  point  of  view  the  matter  may 
be  regarded,  the  parallelism  between  direct  and 
indirect  development  in  Enteropneusta  and 
Onychophora,  not  to  mention  other  cases,  is  a 
very  suggestive  one. 


PALOLO  129 

The  swarming  habit  of  some  marine  Annelids 
for  the  purpose  of  breeding  is  worth  mentioning 
as  affording  a  particularly  interesting  example 
of  bionomical  convergence.  The  celebrated 
palolo  worm  of  Samoa  and  neighbouring  parts 
of  the  South  Pacific  is  the  occasion,  in  Samoa, 
of  an  annual  national  festival  owing  to  the 
regularity  and  abundance  of  its  swarms  and  to 
its  edible  properties.  It  is  eaten  raw  or  baked 
in  leaves  of  the  bread-fruit  tree,  and  the  natives 
send  presents  of  it  to  distant  friends  and  to 
the  chiefs.1  The  most  remarkable  feature  in 
the  biology  of  the  palolo  (Eunice  viridis)  is  its 
perfectly  constant  appearance  in  the  months  of 
October  and  November  when  the  moon  is  in 
its  last  quarter. 

S.  J.  Whitmee,  who  first  reported  upon  this 
phenomenon  in  1875  (P-  Zool.  Soc.,  London,  pp. 
496-502),  found  that  the  palolo  keeps  astro- 
nomical time ;  as  an  indication  of  this  it  will 
suffice  to  quote  the  recorded  fact  that  in  1874 
it  reappeared,  after  an  interval  of  thirteen  lunar 
months,  on  3ist  October  and  ist  November. 
Nineteen  years  later,  in  1893,  Dr  Kramer  col- 
lected the  material  upon  which  A.  Collin  reported, 
again  on  3ist  October  and  ist  November. 

Subsequently  an  Atlantic  palolo  manifestation, 
occurring  in  the  Tortugas  during  June  and  July, 

1  Ant.  Collin,  1897,  "  Bemerkungen  iiber  den  essbaren  Palolo- 
wurm."     In  Kramer's  "  Bau  der  Korallenriffe."     Kiel  und  Leipzig. 

I 


1 30  HABITUDES   AND   ATTITUDES 

on  the  part  of  another  species,  Eunice  fucata, 
was  described  by  A.  G.  Mayer  (1900);  and 
later  still  a  Japanese  palolo,  belonging  to  a 
different  family  (Nereidae),  was  described  under 
the  name  Ceratocephale  osawai  by  Izuka.  The 
last-named  author1  says  that  the  swarming  of 
the  Japanese  worm  takes  place  in  October  and 
November  during  the  nights  closely  following 
the  new  and  the  full  moon,  and  it  resembles 
that  of  the  Atlantic  and  Pacific  species  in  its 
general  course,  but  differs  in  the  circumstance 
that  whereas  in  the  species  of  Eunice  the  sexual 
segments  occupy  the  posterior  portion  of  the 
body  which  becomes  detached  from  the  head- 
end at  the  time  of  swarming,  in  the  Japanese 
worm  the  sexual  segments  are  confined  to  the 
anterior  portion  which,  at  the  swarming  season, 
sheds  the  posterior,  shrunken,  asexual  segments. 

Amongst  the  numerous  methods  of  brood- 
nursing  or  care  of  eggs  and  young,  we  meet 
with  some  extraordinary  cases  of  convergence 
of  varying  degree.  Frequently  they  are  obvious 
enough,  but  that  very  fact  only  serves  to 
strengthen  the  argument  which  I  am  endeavour- 
ing to  develop,  that  convergence  is  not  a  sub- 
ordinate but  a  dominant  factor  in  morphology. 
Firstly,  let  us  take  the  phenomenon  of  buccal 

1  Akira  Izuka,  "  Observations  on  the  Japanese  Palolo,"  Journ. 
Coll.  Sci.  Imp.  Univ,,  Tokyo,  1903,  vol.  xvii.,  article  n,  pp.  i-37>  2 
plates. 


BUCCAL  INCUBATION  131 

incubation,    well   known   in   point   of   time,    but 
not    too    familiar    in    personal    experience.      In 
Ceylon  it  was  first   described   by  the   Rev.    B. 
Boake  (1867-1870)  as  occurring  in  the  Siluroid 
fish  Arius  falcarius,  which  has  been  determined 
by    Dr    Francis    Day   to    be    synonymous   with 
Arius    boakei.      This    fish    frequents    estuarine 
waters   and    is    very   common   in    an    extensive 
backwater  on  the  west  coast   of  Ceylon  called 
the    Panadure    River.     It   has   very  large  yolk- 
laden  eggs  more  than  half  an  inch  in  diameter. 
After  being  laid  by  the  female  they  are  found 
nowhere  except  in  the  mouth  of  the  male,  where 
they  remain   until   they   are   hatched   and   until 
the  young  have  completely  absorbed  the  yolk- 
sac.     I    have    observed    that    the    intestine    of 
brood-nursing   males  is  generally  contracted   to 
narrow    dimensions    and    empty,    a    fact    which 
was  also  noted  by  Day  ;  and  as  generally,  the 
palatine   teeth    of   ovigerous   males   are   greatly 
reduced.     In    one   case,    however,    which    came 
under  my  notice,  where  there  were  fifteen  eggs 
in    the    mouth,    each    containing    an    advanced 
embryo,  the  palatine  teeth  were  not  appreciably 
reduced  and  the  hind-gut  contained  shell-dk#rw. 
The  opening  of  the  oesophagus  is  constricted  and 
virtually  closed,   while  the  mouth  and   pharynx 
are  expanded  to  form  a   spacious  brood-pouch, 
where   the    eggs   and  young   are   exposed   to  a 
constant    current    of   water   which    passes    over 


I32  HABITUDES   AND   ATTITUDES 

them  and  through  the  gill-clefts  of  the  foster- 
parent.  Day  observed  the  same  incubation  in 
the  allied  genus  Osteogeniosus,  and  Dr  Gunther 
and  others  had  recorded  them  for  South  American 
species  of  Arius.1 

Over  against  the  buccal  incubation  of  fishes 
we  may  place  that  of  the  Chilian  toad  -  like 
Batrachian,  Rhinoderma  datwini,  which  was 
regarded  as  one  of  the  most  interesting  finds 
of  the  voyage  of  the  Beagle,  although  it  is  not 
mentioned  in  Darwin's  "  Naturalist's  Voyage." 
The  male  of  this  toad  possesses  a  median  gular 
sac,  representing  an  extension  of  the  buccal 
cavity,  which  opens  by  two  apertures  on  the 
floor  of  the  mouth.  At  the  breeding  season  it 
becomes  a  large  brood-pouch,  lying  freely  in  the 
ventral  lymph-sinus  and  reaching  back  to  the 
pubic  region.  Jiminez  de  la  Espada,  who  first 
brought  this  fact  to  light  in  1872,  found  as  many 
as  fifteen  metamorphosing  larvae  in  the  brood- 
pouch.  Howes2  dissected  a  male  in  1888  which 
contained  eleven  larvae ;  unlike  the  eggs  and  fry 
of  Arius,  these  larvae  were  unequally  advanced, 
only  five  of  them  being  still  provided  with  a  tail. 

Espada  noted  a  shrinkage  of  the  viscera  as  if 
the  foster-parent  had  ceased  to  feed  during  the 

1  In  the  Japan-British  Exhibition  (1910)  specimens  of  a  Nile  fish, 
Tilapia  nilotica^  are  exhibted  by  Mr  C.  L.  Boulenger,  showing 
females  carrying  their  eggs  and  fry  in  the  mouth.     The  eggs  are 
small. 

2  G.  B.  Howes,  "  Notes  on  the  Gular  Brood-Pouch  of  Rhino- 
derma  dariuini"  P.  ZooL  Soc.y  London,  1888,  pp.  231-237. 


CUTANEOUS  INCUBATION  133 

period  of  incubation  as  during  that  of  hiberna- 
tion; but  Howes  found  the  small  intestine  normal 
and  full  of  food-material  in  an  assimilable  condi- 
tion, the  large  intestine  fully  charged  with  excreta, 
and  the  stomach  distended  with  small  beetles  and 
diptera ;  and  he  adds  that  the  alimentary  viscera 
in  general  were  those  of  a  healthy  animal  in  full 
diet.  Howes  thought  that  Espada  was  mistaken, 
but  it  seems  possible  that  both  conditions,  hunger 
diet  and  full  diet,  may  occur. 

In  contrast  with  the*  foregoing  examples  of 
buccal  incubation  on  the  part  of  the  male,  we 
may  quote  cases  of  cutaneous  incubation  on  the 
part  of  the  female,  as  also  in  the  male,  both  in 
fishes  and  batrachians.  In  the  Lophobranchii 
or  Pipe-fishes  the  cutaneous  incubation  of  the 
eggs  by  the  male  attains  a  high  degree  of  per- 
fection within  the  family  Syngnathidae ;  in  the 
allied  family  Solenostomidae  the  ventral  fins 
(absent  in  the  Syngnathidae)  are  enlarged  and 
combine  together  to  form  a  brood-chamber,  with- 
in which  the  eggs  are  borne  upon  cutaneous 
discs,  but  in  this  case  the  female  performs  the 
parental  office.  In  the  Siluroid  genus  Aspredo, 
which  occurs  in  Guiana,  Dr  Giinther  described 
the  remarkable  mode  in  which  the  female  takes 
care  of  her  ova,  carrying  them,  after  oviposition, 
attached  to  the  spongy  integument  of  the  belly, 
as  the  Surinam  toad  Pipa  carries  her  ova  on  the 
back.  But  a  closer  analogy  than  the  Surinam 

I  2 


i34  HABITUDES   AND   ATTITUDES 

toad  is  afforded  by  a  Ceylon  frog  Rhacophorus 
(Polypedates]  reticulatus  as  described  by  Giinther.1 
Here  the  ova,  about  twenty  in  number,  were 
found  attached  to  the  abdomen  of  the  female, 
and  when  detached  they  adhered  firmly  together 
so  as  to  form  a  flat  disc.  In  the  European  Mid- 
wife toad,  Alytes  obstetricans,  the  mode  of  nursing 
is  analogous  to  that  of  Rhacophorus  reticulatus, 
except  that  in  Alytes  it  is  the  male  that  takes 
care  of  the  spawn. 

In  illustration  of  the  widespread  nature  of  the 
phenomenon  of  convergence  may  be  mentioned 
a  case  of  brood-nursing  on  the  part  of  some 
insects.  The  family  of  amphibious  water-bugs, 
Belostomatidae,  is  noted  for  the  possession  of 
the  habit  of  carrying  the  eggs  in  the  form  of 
a  disc  cemented  upon  the  back  of  the  male,  to 
which  they  are  attached  by  the  female.2  A 
species  of  this  family  where  the  habit  can  be 
observed  with  comparative  ease  is  Sphcerodema 
rusticum  Fabr.,  which  occurs  in  the  tanks  of 
Ceylon  amongst  the  weeds  at  the  margin.  The 
eggs  remain  on  the  back  until  the  young  hatch 
out  into  the  water. 

Amongst  fresh  -  water  fishes  many  cases  of 
convergence  are  found  in  respect  of  the  manner 
in  which  they  prepare  their  nests  and  deposit 
their  eggs.  One  pair  of  examples  will  suffice, 

1  Ann.  Nat.  Hist.,  May,  1876,  p.  379. 

2  Cf.  W.  L.  Distant,  "Rhynchota,"  vol.  iii.,  Fauna  Brit. 
1906,  p.  34. 


FLOATING  EGGS  135 

namely,  the  Indian  murral,  Ophiocephalus  striatus? 
as  compared  with  the  American  bowfin,  Amia 
calva,  species  which  are  far  enough  apart  in  the 
systematic  scale,  the  former  a  Teleostean,  the 
latter  a  Ganoid.  Both  make  circular  clearings 
in  the  shallow  water  of  lakes  or  irrigation  tanks, 
biting  away  the  surrounding  weeds ;  and  in  both 
cases  the  male  parent  tends  the  nest,  lying  in 
wait  in  special  runways.  But  whereas  the  eggs 
of  Amia  are  strewn  over  the  bottom  of  the  nest, 
those  of  the  murral  float  in  a  single  layer  at 
the  surface,  in  contact  with  one  another,  but  not 
adhering  together.  This  exceptional  behaviour 
of  the  eggs  of  a  fresh-water  fish  is  shared  by 
those  of  another  species  of  the  same  genus, 
Ophiocephalus  punctatus. 

These  floating  eggs  of  Ophiocephalus  owe  their 
buoyancy  to  the  presence  of  a  single  large  oil- 
globule,  which  occupies  the  greater  part  of  the 
bulk  of  the  ovum  and  is  immersed  in  the  amber- 
coloured  yolk  adjacent  to  the  uppermost  pole 
of  the  egg.  The  eggs  thus  come  to  lie  immedi- 
ately below  the  surface  film  of  water,  exposed 
to  the  quickening  influence  of  air  and  sun,  and 
protected  thereby  from  the  attacks  of  fungi,  to 
which  they  are  extremely  liable  as  soon  as 
the  conditions  of  existence  fall  below  a  certain 

1  A.  Willey,  "  Observations  on  the  Nests,  Eggs,  and  Larvae  of 
O.  striatus?  Spolia  Zeylanica,  vol.  vi.,  1909]  (1910),  pp.  108-123. 
The  comparison  with  Amia  is  based  on  the  description  given  by 
Professor  Bashford  Dean. 


136  HABITUDES   AND   ATTITUDES 

optimum.  For  three  days  after  hatching  the 
larvse  remain  at  the  surface,  floating  on  one  side 
with  yolk  -  sac  well  up.  Before  hatching,  the 
body  of  the  embryo  encircles  about  two-thirds 
of  the  equatorial  region  of  the  yolk  like  a  belt  ; 
the  tail  then  twitches,  the  vitelline  membrane  is 
ruptured,  and  the  larva  with  its  yolk-sac  is  set 
free. 

The  floating  eggs  of  these  tropical  fresh-water 
fishes  are  not  comparable  with  the  hyaline 
pelagic  eggs  of  many  marine  fishes  because  they 
do  not  move  from  the  nest ;  their  buoyancy  is  part 
of  the  method  of  nidification  and  brooding,  not 
a  means  of  dispersal.  I  do  not  know  any  other 
instances  of  eggs  of  fresh-water  fishes  floating  at 
the  surface  of  the  water  by  their  own  buoyancy  ; 
but  the  same  advantages — proximity  to  atmos- 
pheric air  and  to  sunlight — are  secured  in  other 
ways,  as  by  attachment  to  aquatic  plants,  or  by 
deposition  in  very  shallow  water,  or  by  special 
methods  of  nidification,  as  in  the  floating  nests 
of  Gymnarchus  or  the  foam  nests  of  Sarcodaces, 
which  were  described  by  the  late  J.  S.  Budgett.1 
There  would  seem,  however,  to  be  points  of  closer 
comparison  between  the  eggs  of  the  murral  and 
those  of  some  marine  fishes,  namely,  the  weevers 
(Trachinidse),  according  to  the  observations  of 
J.  Boeke  (1903).  These  eggs  are  maintained 

1  Cf.  The  Budgett  Memorial  Volume,  edited  by  J.   Graham 
Kerr,  Cambridge  Univ.  Press,  1907. 


YOLK-SACS  137 

at  the  surface  of  the  sea  by  means  of  a  single 
large  oil-globule  ;  four  or  five  days  after  oviposi- 
tion  the  embryos  are  hatched ;  four  or  five  days 
after  hatching  the  yolk  has  become  absorbed. 
The  buoyancy  of  the  yolk-sac  causes  the  larvae 
to  float  helplessly  for  some  time  after  hatching, 
with  the  yolk-sac  uppermost. 

The  accumulation  of  yolk  in  eggs,  or  the 
lecithality  of  the  ovum,  also  offers  numerous 
examples  of  convergence  as  between  cartilaginous 
fishes,  bony  fishes,  and  sauropsida  (birds  and 
reptiles)  as  well  as  amongst  invertebrate  animals. 
The  large  yolk-sacs  of  Arius  and  of  Gymnarchus 
have  no  genetic  relation  to  each  other  nor  to 
those  of  sharks  and  rays.  The  yolk  -  sac  of 
cephalopod  molluscs  is  a  structure  sui  generis^ 
and  it  is  particularly  noteworthy  that  the  most 
ancient  existing  genus,  Nautilus,  has  the  most 
macrolecithal  egg  of  all  whose  eggs  are  known. 


CHAPTER   IX 

THE  WAYS  OF  BREATHING 
(RESPIRATORY  CONVERGENCE) 

THE  factors  which  combine  to  produce  a  structural 
or  organic  unit  in  the  animal  body  which  will  be 
fixed  by  inheritance  are  inconceivably  complex. 
Nevertheless,  we  have  seen  that  essentially  the 
same  combination  can  be  repeated  independently 
in  different  cases.  The  present  state  of  know- 
ledge justifies  the  provisional  assertion  that  the 
higher  combination  which  leads  to  the  establish- 
ment of  an  animal  form  possessing  the  essential 
component  elements  of  a  definite  morphological 
type,  cannot  be  repeated.  The  theory  of  con- 
vergence is  therefore  not  calculated  to  precipitate 
us  into  morphological  chaos,  howsoever  startling 
its  manifestations  may  be. 

A  great  deal  might  be  written  upon  the  large 
subject  of  respiratory  convergence,  but  I  will 
touch  upon  it  briefly.  Respiration  is  one  of 
the  primary  properties  of  living  matter,  and  the 
general  principle  which  governs  the  mechanism 
of  respiration,  namely,  the  diffusion  of  gases 
(oxygen  and  carbon  dioxide)  through  moist 

138 


MECHANISM   OF  RESPIRATION  139 

membranes  or  across  moist  surfaces,  is  common 
to  all  animals  and  even  to  plants  as  well ;  but 
the  special  mechanism  of  respiration  exhibits 
great  phyletic  diversity.  There  are  four  or  five 
principal  methods  of  breathing :  cutaneous,  by 
the  entire  surface  of  the  body,  as  with  earth- 
worms, leeches,  and  planarians ;  branchial,  by 
specialised  cutaneous  processes  called  gills  or 
branchiae,  as  with  many  marine  Annelid  worms, 
some  leeches  (Branchellion  and  Ozobranchus], 
molluscs,  and  Crustacea ;  tracheal,  by  air-tubes 
traversing  the  body  and  surrounding  the  viscera, 
as  with  insects  and  spiders ;  trematic,  by  gill- 
clefts  or  visceral  clefts  piercing  the  body-wall 
and  leading  from  the  cavity  of  the  pharynx  or 
anterior  part  of  the  alimentary  canal  to  the 
exterior ;  pulmonary,  by  lungs  or  air-chambers, 
as  with  pulmonate  molluscs  (Gastropods)  and 
air-breathing  vertebrates. 

These  different  methods  of  respiration  are  not 
merely  adaptations  to  the  environment,  but  they 
are  adaptations  which  keep  pace  with  evolution 
independently  of  the  environment.  The  super- 
ficial resemblance  in  shape  of  body,  swimming 
movements,  and  burrowing  habits,  between  the 
Annelid  worm  Ophelia,  which  breathes  by  simple 
filiform,  cutaneous  gills,  and  Amphioxus,  which 
breathes  by  gill-clefts,  has  been  remarked  by 
Cav.  Lo  Bianco  and  by  myself;  they  are  often 
taken  together  in  the  same  sandy  bottom,  in  the 


i4o  THE   WAYS   OF   BREATHING 

Mediterranean  and  in  the  Indo-Pacific.  The 
environment  is  identical ;  the  adaptations  are 
different  according  to  their  respective  grades  of 
evolution  and  lines  of  descent.  Again,  in  many 
ways  the  Annelida  are  more  highly  organised 
than  the  Enteropneusta,  though  the  latter  possess 
gill-clefts. 

An  equal  degree  of  resemblance  based  on  con- 
vergence between  members  of  distinct  though 
allied  families  is  that  of  the  Jumping  Blenny, 
Salarias,  and  the  Jumping  Goby,  Periophthalmus. 
These  fishes  habitually  come  out  of  the  water, 
the  former  to  lounge  and  skip  about  rocks,  the 
latter  upon  mud  and  mangrove  roots ;  they  both 
have  large  goggle-eyes  and  both  can  leap  about 
as  a  normal  mode  of  progression  out  of  water, 
while  the  jumping  goby  can  also  ricochet  over 
the  surface  of  the  water.  As  described  by 
Moseley1  and  again  by  Hickson,2  Periophtkalmus 
jumps  both  out  of  water  and  on  the  surface  by 
means  of  the  bent,  muscular,  pectoral  fins,  of 
course  assisted  by  the  tail.  Salarias,  which  is 
common  on  certain  parts  of  the  coast  of  Ceylon, 
performs  surprising  leaps  by  the  action  of  its 
tail,  which  is  kept  curved  when  on  the  ground 
ready  for  a  spring.  Any  one  familiar  with  Peri- 
ophthalmus, which  is  one  of  the  everyday  sights 
in  suitable  localities  in  the  Eastern  Tropics, 

1  H.  N.  Moseley,  "  Notes  by  a  Naturalist  on  H.M.S.  Challenger? 
second  edit.    London,  1892. 

S.  J.  Hickson,  "  A  Naturalist  in  North  Celebes."   London,  1889. 


AIR-BREATHING   FISHES  141 

would  be  certain  to  mistake  Salarias  for  it 
unless  otherwise  instructed.  When  out  of  water 
the  opercular  membrane  is  kept  closely  pressed 
against  the  body  behind  the  gill-opening,  so  that 
the  gill  -  cavity  is  temporarily  converted  into  a 
virtual  lung-chamber. 

Other  fishes  which  can  progress  out  of  water 
on  suitable  ground  without  falling  on  to  one  side 
areAnabas,  Clarias,  Saccobranchus,  Ophiocephalus, 
and  others.  These  may  all  be  designated  walk- 
ing fishes.  Anabas  helps  itself  along  by  means 
of  its  opercular  spines,  Clarias  and  Saccobranchus 
by  their  pectoral  spines,  Ophiocephalus  by  move- 
ments of  its  flattened  head,  assisted  by  flexions 
of  body  and  tail.  All  of  these  fishes  can  and 
must  breathe  air  by  means  of  special  growths  or 
diverticula  connected  with  the  upper  division  of 
the  gill-cavity  above  the  gill-clefts.  Clarias  and 
Saccobranchus  are  both  Siluroids,  but  the  coral- 
like  dendritic  appendages  in  the  suprapharyngeal 
chambers  in  Clarias  are  much  more  like  the 
lamelliform  labyrinthine  organs  in  Anabas  than 
the  diverticula  of  Saccobranchus.  If  these  air- 
breathing  Teleostean  fishes  are  prevented  from 
reaching  the  surface  in  order  to  take  the  air,  they 
become  drowned ;  and  this  habitual  aeropneustic 
function  of  the  accessory  branchial  organs  was, 
when  first  demonstrated,  regarded  by  Professor 
Huxley  as  a  great  fact.1  Saccobranchus  will  live 

1  Rev.  Barcroft  Boake,  "  On  the  Air-Breathing  Fish  of  Ceylon," 
Journ.  Ceylon  Branch  Roy.  Asiat.  Soc.t  iv.,  1867-1870. 


I42  THE   WAYS  OF   BREATHING 

longer  out  of  water  than  it  will  when  kept  in 
water  and  deprived  of  access  to  the  surface, 
but  it  cannot  withstand  desiccation,  nor  will  it 
voluntarily  leave  the  water  when  the  latter  is 
polluted  ;  if  its  excursions  to  the  surface  are  not 
sufficient  to  counteract  the  poisonous  effects  of 
the  gaseous  exhalations  from  the  muddy  bottom 
of  a  stagnant  pool,  it  will  perish  miserably,  some- 
times coincidently  with  its  last  gulp  of  air,  as 
I  have  witnessed. 

The  above-named  fishes  are  amphipneustic 
without  being  amphibious,  and  in  this  respect 
are  comparable,  by  way  of  convergence,  with  the 
fresh- water  pulmonate  Mollusca  (Limnseidse),  and 
with  the  Dipnoan  fishes  or  Dipnoi ;  whilst  the 
latter  are  comparable,  by  way  of  homology,  with 
the  Amphibia.  The  respiratory  movements  of 
the  legless  batrachian,  Ichthyophis,  which,  with 
the  loss  of  appendages,  has  still  retained  many 
primitive  features,  involve  the  entire  branchial 
or  pharyngeal  region  including  the  throat,  rapid 
contractions  shimmering  over  from  the  ventral  to 
the  dorsal  surface,  while  the  nasal  orifices  are 
kept  perpetually  open.  The  original  branchial 
region  is  clearly  marked  off  like  a  collar  from 
the  rest  of  the  body  during  life  (Fig.  12),  and 
the  impression  conveyed,  which  is  in  accord- 
ance with  its  known  anatomical  structure,1  is 

1  Cf.  the  elaborate  monograph  by  Drs  P.  and  F.  Sarasin.  The 
incubation  of  its  eggs,  the  parent  coiling  round  the  egg-clump, 
resembles  that  of  a  Python. 


ICHTHYOPHIS  143 

that  while  the  essential  organs  of  respiration 
have  changed  from  gill  -  clefts  to  lungs,  the 
muscular  and  skeletal  mechanism  of  respiration 
has  remained  practically  unchanged.  It  may  be 
added  that  Ichthyophis  is  a  good  swimmer  but 
tries  to  get  out  of  water  as  soon  as  possible, 
creeping  with  difficulty  on  a  free  surface  by 
serpentine  jerks,  but  moving  easily  through 
narrow  crevices  where  its  contact  requirements 
(i.e.,  stereotropism)  are  satisfied.  Similar  obser- 
vations with  regard  to  the  respiratory  movements 
have  been  made  by  my  friend  Professor  Graham 
Kerr  (in  Hit.}  on  the  Dipnoan  fish  Lefidosiren.1 


FIG.  1 2.  Head  and  fore-body  of  Ichthyophis glutinosus  from  above, 
to  show  the  collar-like  respiratory  region.  At  the  sides  of 
the  head  in  front  of  the  eyes  is  the  pair  of  peculiar  retractile 
tentacles. 

The  fact  that  in  the  course  of  the  substitu- 
tion of  lungs  for  gills  the  respiratory  movements 

1  For  a  discussion  of  the  nerve-supply  see  J.  Graham  Kerr, 
"Note  on  Swim-Bladder  and  Lungs,"  Proc.  Roy.  Physical  Soc.t 
Edinburgh,  vol.  xvii.,  1908,  pp.  170-174. 


144  THE   WAYS   OF   BREATHING 

are  continued  without  interruption  from  the  one 
to  the  other  system,  shows  how  the  transition 
could  be  effected,  but  it  does  not,  in  my  opinion, 
necessarily  support  Spengel's  and  Goette's  theory 
of  the  origin  of  vertebrate  lungs  and  air-bladders 
from  gill-pouches.1  This  is  an  interesting  theory, 
and  adds  weight  to  the  conception  of  the  gill- 
cleft  as  an  autonomous  "morphon,"  but  there  are 
difficulties  in  the  way  of  its  acceptance.  In  the 
first  place,  from  the  analogy  of  hydrostatic  organs 
in  other  phyla,  it  does  not  seem  necessary  to 
invent  a  factitious  explanation  of  the  air-bladder 
as  a  gill-pouch  derivative ;  the  accumulation  of 
air  or  gas  in  air-chambers  is  a  widely  distributed 
phenomenon,  and  in  some  fishes  (e.g.,  the  Globe 
Fishes)  the  oesophagus  itself  can  be  converted 
into  an  air-chamber  at  the  will  of  the  animal. 

There  seems  to  be  no  particular  difficulty  in 
supposing  that  the  air-bladder  arose  as  a  single 
or  paired  diverticulum  of  the  fore-gut  for  the 
storage  of  air;  and  whether  the  first  function 
was  respiratory  or  hydrostatic  or  both,  is  another 
question.2  Spengel  lays  great  stress  upon  the 

1  J.  W.  Spengel,  "  Ueber  Schwimmblasen,  Lungen  und  Kiemen- 
taschen  der  Wirbelthiere,"  Zool.  Jahrb.  Suppt.,  1904,  pp.  727-749. 
In  this  paper  the  term  "morphon"  is  suggested;  it  means  a 
morphological  unit  or  element. 

1  Facts  in  support  of  the  view  that  the  original  function  of  the 
air-bladder  was  respiratory  were  brought  forward  in  a  paper  by 
Charles  Morris  on  "  The  Origin  of  Lungs,  a  Chapter  in  Evolution," 
in  The  American  Naturalist,  vol.  xxvi.,  1892,  pp.  975-986.  This 
opinion  is  tentatively  supported  in  principle  by  Hubrecht  (loc.  «"/., 
1908),  who  refers  to  Assheton's  paper  on  "  Gymnarchus "  in  the 
Budgett  Memorial  Volume. 


MEGALOPS  145 

fact  that  the  arteries  which  supply  the  lungs  of 
Dipnoi  and  the  air-bladder  of  some  other  fishes 
(e.g.,  Polypterus)  and  the  lungs  of  all  air-breathing 
vertebrates  from  the  Amphibia  onwards,  arise  as 
branches  of  the  fourth  afferent  branchial  arteries. 
But  Burne x  has  shown  that  the  accessory  branchial 
diverticula  or  air-pouches  of  Saccodranckus,  which 
penetrate  the  body  musculature  throughout  the 
greater  part  of  the  length  of  the  trunk  from  the 
gill-cavity  to  the  tail,  lying  above  the  transverse 
processes  on  either  side  of  the  vertebral  column, 
are  also  vascularised  from  the  fourth  afferent 
branchial  artery  on  each  side ;  but  these  air- 
pouches  co-exist  with  a  true  air-bladder.  We 
have,  in  fact,  in  this  case  a  very  delicate  example 
of  vascular  convergence. 

The  air-bladder  of  the  estuarine  Clupeoid  fish 
Megalops  cyprinoides,  common  in  Ceylon,  is  pro- 
vided on  its  inner  surface  with  an  abundance 
of  spongy,  vascular,  alveolar  proliferations  which 
are  especially  dense  just  behind  the  large  orifice 
by  which  it  communicates  directly,  without  the 
intermediation  of  a  pneumatic  duct,  with  the 
oesophagus  immediately  behind  the  gill  -  clefts 
dorsally ;  it  is  not  vascularised  from  the  branchial 
arches,  but  this  species  can  live  for  a  long 
time  after  capture,  an  uncommon  feature  in  the 

1  R.  H.  Burne,  "  On  the  Aortic  Arch  System  of  Saccobranchus 
fossilis?  Journ.  Linn.  Soc.  Zool^  xxv.,  1894,  pp.  48-55. 

K 


146  THE   WAYS   OF   BREATHING 

herring   family  and  contrary  to    the   experience 
of  the  common  herring. 

The  Siluroid  fish  Arius  falcarius,  of  which 
mention  has  been  made  above,  exhibits  great 
viability  out  of  water.  A  large  female,  14^ 
inches  long,  which  had  been  caught  by  a  hook 
but  had  not  been  injured,  lived  for  nearly  five 
hours  out  of  water,  breathing  regularly  by 
mouth  and  gills,  without  accessory  structures, 
closing  the  gill  -  cavity  behind  by  the  opercular 
membrane.  After  it  had  been  out  of  water  for 
about  an  hour  I  counted  80  buccal  respirations 
to  the  minute,  the  opercular  membrane  beating 
time  with  the  mouth  but  remaining  quite  closed 
behind.  After  two  hours  there  were  70  respira- 
tions to  the  minute  ;  and  after  three  hours  60 
feebler  respirations,  the  opercular  membrane  now 
commencing  to  gape  behind.  At  the  end  of 
the  experiment  the  heart  was  removed  and  con- 
tinued beating  outside  the  body.  This  example 
shows  that  the  phenomenon  of  viability  out  of 
water  is  something  apart  from  the  possession 
of  accessory  branchial  organs. 

Another  Siluroid  fish,  Plotosus  canius,  exhibits 
similar  viability,  living  from  early  morning  until 
afternoon  (over  six  hours)  out  of  water,  the  gills 
remaining  quite  fresh  to  the  end.  In  this  case 
periodical  expirations  took  place  through  the 
gill-opening ;  every  now  and  then  the  opercular 
membrane  was  raised  several  times  in  succession, 


PLOTOSUS  147 

and  this  was  followed  by  a  number  of  buccal  in- 
spirations without  the  gill-covers  being  reflected. 
The  raising  of  the  gill-covers  coincides  with  the 
taking  of  a  deep  breath.  I  counted  first  32  in- 
spirations and  13  expirations  in  a  minute,  then 
32  inspirations  to  10  expirations  ;  a  third  count- 
ing gave  30  and  n,  a  fourth  29  and  12.  The 
expirations  were  not  evenly  distributed,  some- 
times 5  or  6  occurring  together.  These  observa- 
tions show  the  great  physiological  importance  of 
the  opercular  membrane  whose  systematic  worth 
is  sometimes  regarded  as  nil. 

The  typical  Molluscan  gill  was  named  the 
ctenidium  by  Lankester1  in  order  to  fix  its 
morphological  independence  and  to  distinguish 
it  from  other  gills  of  invertebrates  such  as  the 
Annelid  parapodial  gill,  the  Crustacean  arthro- 
podial  gill,  the  Limuloid  gill-book,  and  others, 
as  well  as  from  secondary  Molluscan  pallial  gills, 
as  in  the  Prosobranchiate  genus  Patella  (limpet) 
and  in  the  Opisthobranchiate  genera  Phyllidia 
and  Pleurophyllidia.  Thus  the  organs  of  re- 
spiration which  take  the  form  of  cutaneous  gills 
are  not  homologous  throughout ;  in  other  words, 
they  are  not  monophyletic  but  polyphyletic  in 
nature  and  origin  ;  but  the  particular  structure 
known  as  the  ctenidium  with  which  a  special 
sensory  apparatus,  the  osphradium  and  its  nerve- 
supply,  is  associated,  is  homologous  throughout 

1  Zoological  Articles  (Encyc.  Brit.\  Edinburgh,  1891. 


148  THE   WAYS  OF   BREATHING 

the  Molluscan  phylum,  that  is  to  say,  the  cten- 
idium  is  monophyletic. 

The  spirally  thickened  air  -  tubes  or  tracheae 
of  Myriopods,  Insects,  and  Spiders  bear  so  close 
a  correspondence  in  structure  and  function,  rami- 
fying through  the  body  like  blood-vessels,  but 
effecting  the  circulation  of  air  instead  of  blood, 
that  all  the  air  -  breathing  Arthropods  were 
formerly  classed  together  as  Tracheata  in  con- 
tradistinction to  the  aquatic  Arthropods  which 
were  called  Branchiata. 

For  more  than  a  quarter  of  a  century  it  has 
been  recognised  that  the  Tracheate  Arthropods 
could  not  be  reduced  to  a  common  standard,  and 
it  has  also  come  to  be  realised  that  the  tracheae  of 
Insects  and  Arachnids  have  had  separate  origins, 
and  are  therefore  different  morphologically  though 
similar  histologically  and  physiologically. 

This  fundamental  example  of  tracheal  con- 
vergence is  rendered  more  remarkable  by  the  fact 
that  even  within  the  limits  of  the  Arachnoid  sub- 
phylum,  the  tracheae  have  had  at  least  a  twofold 
origin,  namely,  from  lung-books  and  from  ecto- 
dermal  tendons;  so  that  "similarity  of  structure 
in  the  fully  developed  tracheae  does  not  mean 
similarity  of  origin  "  [Purcell].  Pureell  has  shown 
that  the  tendinal  or  medial  tracheal  trunks  in 
Dipneumonous  spiders  are  equivalent  in  their 
entirety  to  metamorphosed  entapophyses  (ecto- 
dermal  tendons) ;  the  lateral  tracheal  trunks,  on 


TRACHEAL   CONVERGENCE  149 

the  contrary,  are  serially  homologous  with  the 
pulmonary  sacs  of  the  preceding  somite,  and 
actually  homologous  with  the  second  pair  of 
pulmonary  sacs  in  Tetrapneumonous  spiders.1 

Purcell  affirms  that  the  arguments  in  favour  of 
the  branchial  origin  of  the  lung-books  of  spiders, 
advocated  by  Lankester  in  1881,  appear  over- 
whelming. An  interesting  analogy  may  there- 
fore be  drawn  between  the  Pulmonate  Arachnida 
and  the  Pulmonate  Mollusca,  where  a  lung- 
chamber  has  likewise  been  substituted  for  a 
gill-chamber ;  and  just  as  in  Arachnida,  accord- 
ing to  Pocock  (1893),  there  is  reason  to  believe 
that  tracheal  tubes  have  replaced  lung-books  at 
least  twice,  namely,  in  the  Dipneumones  and  in 
the  Pseudoscorpiones ;  so  in  the  Land  Mollusca 
a  pulmonary  chamber  has  twice  replaced  a  gill- 
chamber,  namely,  in  the  Land  Operculates,  which 
belong  to  the  order  Prosobranchiata,  and  in  the 
inoperculate  Pulmonata,  which  are  related  to  the 
Opisthobranchiata. 

The  internal  tracheae  (of  cutaneous  origin) 
are  not  homologous  throughout  the  Arthropod 
phylum — they  are  polyphyletic  like  the  external 
cutaneous  branchiae  of  soft-bodied  invertebrates. 

Now  with  regard  to  the  trematic  mode  of 
aquatic  respiration,  or  breathing  by  means  of 
gill-clefts,  the  question  naturally  arises  whether 

1  W.  F.  Purcell,  "  Development  and  Origin  of  the  Respiratory 
Organs  in  Araneae,"  Quart.  Journ.  Micr.  Sc.>  vol.  liv.,  1909,  pp. 
l-no  ;  contains  full  bibliography. 

K    2 


i  So  THE  WAYS  OF   BREATHING 

the  latter  are  another  generalised  form  of  re- 
spiratory organ  which  may  have  arisen  de  novo 
in  different  divisions  of  the  animal  kingdom,  or 
whether  gill-clefts  are  not  rather  a  specialised 
formation,  homologous  throughout,  like  the 
Molluscan  ctenidium.  The  analogies  of  the 
cutaneous  branchiae  and  tracheae  render  the 
question  a  legitimate  one  for  discussion  and 
an  extremely  difficult  one  to  settle.  Many 
zoologists,  of  whom  I  happen  to  be  one,  think 
that  the  gill -cleft  is  a  monophyletic  structure, 
and  in  order  to  give  effect  to  this  point  of 
view  I  suggested,  as  a  memoria  technica,  the 
phylogenetic  term  Branchiotrema,1  to  include  all 
animals  possessing  gill-clefts  at  any  period  of 
their  life-history. 

The  extraordinary  persistence  of  gill -clefts 
and  gill-pouches  in  the  embryos  of  the  higher 
lung-breathing  vertebrates,  the  constancy  of  their 
innervation  in  all  Craniate  vertebrates,  and  the 
combination  of  structures  which  accompany  them 
in  all  Chordates,  speak  for  their  homogeneity. 
Whilst  we  may  admit  that  the  general  homology 
of  gill-clefts  is  open  to  question,  we  must  at  the 
same  time  assert  that  the  burden  of  proof  to  the 


1  Cf.  A.  Willey,  "  Enteropneusta  from  the  South  Pacific,"  Zoo- 
logical Results^  part  iii.,  Cambridge,  1899,  pp.  223-334.  See  also 
R.  C.  Punnett's  memoir  on  the  Enteropneusta  from  the  Maldive 
and  Laccadive  Islands  in  Stanley  Gardiner's  "  Fauna  and  Flora 
of  the  Maldive  and  Laccadive  Islands,"  1904-1905. 


GILL-CLEFTS  151 

contrary  rests  with  those  who  deny  it.  Up  to 
the  present  there  has  been  no  brilliant  demonstra- 
tion of  the  diphyletic  origin  of  gill-clefts.  The 
assumed  homology  between  the  gill-clefts  of  the 
Enteropneusta  and  those  of  Amphioxus  has  been 
doubted  because  their  histological  composition  is 
not  identical ;  but  we  know  now  that  histological 
identity  is  no  safer  guide  to  morphology  than  is 
anatomical  identity.  This  is  where  the  matter 
rests  at  present  with  regard  to  trematic  respira- 
tion. The  differences  between  the  gill-clefts  of 
the  Enteropneusta  where  the  tongue-bar  is  the 
principal  component,  and  the  gill-clefts  of  Amphi- 
oxus where  the  tongue-bar  is  a  secondary  com- 
ponent, are  such  as  one  would  be  prepared  to 
find  in  allied  groups  which  present  unequal  grades 
of  organisation. 

We  have  already  mentioned  several  examples 
of  convergent  pulmonate  respiration,  and  it  only 
remains  to  be  added,  for  the  sake  of  complete- 
ness, that  the  lungs  of  the  higher  vertebrates 
are  acknowledged,  on  grounds  of  comparative 
anatomy,  to  be  homologous  with  the  air-bladder 
of  Teleostean  fishes  in  spite  of  differences  in  the 
vascularisation  and  topography  of  these  organs. 

According  to  Owen  (Preface  to  Hunter's 
Animal  Oeconomy,  1837),  Harvey  was  the  first 
to  compare  the  abdominal  air-sacs  of  the  bird 
with  those  of  reptiles  and  fishes ;  and  in  this  he 
was  followed  by  John  Hunter,  who  discovered  the 


152  THE  WAYS  OF   BREATHING 

"  air-cells  "  in  the  bones  and  muscular  interstices 
of  birds  in  1774,  simultaneously  with  Camper. 
The  abdominal  air-sacs  of  birds  are  appendages 
of  the  lungs,  and  although  there  exists  a  general 
homology  between  lungs  and  the  air -sac  of 
fishes,  there  is  no  special  homology  between  the 
abdominal  extensions  of  the  lungs  in  birds  and 
the  latter. 


CHAPTER  X 

CONVERGENCE  IN  MINUTE  STRUCTURE 
(HISTOGENETIC  CONVERGENCE) 

IN  the  preceding  chapter  we  have  seen  that 
histogenetic  convergence  gives  no  clue  to  affinity, 
and  that  histogenetic  divergence  is  no  proof  of 
want  of  affinity.  We  may  now  go  further  than 
this  and  add  that  where  we  do  find  actual  histo- 
logical  identity,  as  between  members  of  different 
phyla,  it  seems  certain  that  we  are  in  the  presence 
of  true  convergence  in  the  sense  in  which  that 
term  is  employed  here ;  and,  in  the  light  of  facts 
which  are  now  available,  it  even  begins  to  appear 
strange,  although  only  a  matter  of  a  few  years 
or  months  ago,  that  histological  identity  should 
ever  have  been  insisted  upon  as  a  criterion  of 
homology  except  within  well-defined  limits. 

To  my  thinking,  one  of  the  most  remarkable 
examples  of  histogenetic  convergence  is  that  of 
the  excretory  organs  of  Amphioxus  as  com- 
pared with  those  of  certain  Annelid  worms. 
The  excretory  tubules  of  Amphioxus  were  dis- 
covered independently  by  Weiss  and  Boveri 
in  1890  and  were  described  in  detail  by  the 


154    CONVERGENCE   IN   MINUTE   STRUCTURE 

latter  in  1892.  Attached  in  clusters  to  the 
walls  of  the  tubules  Boveri  found  long  pin- 
shaped  cells,  which  he  called  "  Fadenzellen " 
(thread-cells),  projecting  into  the  dorsal  coelom. 
They  appeared  to  consist  of  a  small  roundish 
cell-body  mounted  upon  a  long  thread-like  stalk 
which  was  inserted  into  the  tubule.  Ten  years 
later  Goodrich,  who  had  been  making  some 
notable  discoveries  in  regard  to  the  structure  of 
the  nephridia  of  Polychsete  worms,  turned  his 
attention  to  the  tubules  of  Amphioxus  and 
promptly  discovered  that  Boveri's  thread-cells 
are  identical  with  the  solenocytes  which  he  had 
found  in  Polychaeta. 

Goodrich  showed  that  Boveri's  thread  -  like 
stalk  is  really  a  slender  hollow  tube  of  great 
length,  carrying  the  cell  -  body  floating  in  the 
coelom,  or  adhering  to  the  adjacent  walls  of 
the  coelom,  at  its  distal  end,  and  opening  into 
the  renal  tubule  at  the  proximal  end.  "  A  long 
flagellum  attached  at  its  base  to  the  cell  placed 
at  the  end  of  the  tube  works  rapidly  down  the 
tube  and  far  into  the  excretory  canal."1  Goodrich 
observes  that  the  excretory  organs  of  Amphioxus 
and  the  nephridia  of  Phyllodoce  are  in  all  essentials 
identical,  and  he  adds  that  "  if  two  such  excretory 
organs  as  the  solenocyte  -  bearing  nephridia  of 
Phyllodoce,  and  the  solenocyte-bearing  kidneys 

1  E.  S.  Goodrich,  "On  the  Structure  of  the  Excretory  Organs  of 
Amphioxus,"  Part  i.,  Quart.  Jour.  Micr.  Sc.^  vol.  xlv.,  1902,  pp- 
493-501  ;  Part  ii.  Ibid.  vol.  liv.,  1909,  pp.  185-205. 


SOLENOCYTES  155 

of  Amphioxus  could  be  shown  to  have  been  inde- 
pendently evolved,  we  should  have  to  give  up 
structural  resemblance  as  a  guide  to  homology." 
He  says  in  a  footnote  that  the  only  case  which 
seems  to  him  at  all  comparable  is  that  of  the 
nematocysts  in  Coelenterates,  Planarians,  and 
Molluscs.  To  this  case  we  may  add  the 
myoepithelial  cells  in  Coelenterates,  Nematodes, 
and  Tunicata ;  and,  as  Goodrich  admits,  the 
flame-cells  of  flatworms,  Rotifers,  and  Polyzoa 
(Entoprocta)  are  probably  of  the  same  nature 
as  his  solenocytes. 

I  venture  to  interpret  Goodrich's  discovery 
as  a  brilliant  demonstration  of  histogenetic  con- 
vergence and  submit  the  following  explanation, 
basing  the  argument  on  personal  conviction,  on 
an  appeal  to  known  facts,  and  on  certain  general 
considerations  : — 

Just  as  various  offices  are  constituent  parts 
of  the  body  politic,  so  various  organs  are  con- 
stituent parts  of  the  animal  economy.  Pharynx, 
oesophagus,  crop,  gizzard,  stomach,  liver  and 
other  diverticula,  intestine  and  rectum,  are 
constituent  parts  of  the  digestive  tract  of  a 
coelomate  animal  and  are  likely  to  appear  when 
called  into  requisition  by  the  necessities  of 
adaptation  and  evolution  ;  and  equally  likely  to 
disappear  when  not  specially  required.  I  may 
refer  here  to  the  remarkably  simple  digestive 
tract  of  the  Scombresocidse  (e.g.,  B clone),  where 


156     CONVERGENCE   IN    MINUTE  STRUCTURE 

the  oesophagus  is  suppressed,  the  outward  differ- 
entiation of  the  stomach  is  lacking,  and  there 
is  no  pylorus  and  no  pyloric  cceca.  So  again, 
continuing  our  analysis,  we  find  that  flame-cells 
or  solenocytes,  nematocysts,  myoepithelial  cells, 
spicule-forming  cells,  ganglion  cells,  pigment  cells, 
striated  muscle  cells,  etc.,  may  be  regarded  as 
belonging  to  the  category  or  repertory  of  the 
primary  constituent  elements  of  animal  tissues, 
and  will  appear  when  required  to  fit  in  with 
a  special  set  of  physiological  conditions.  Let 
us  imagine  a  limited  number  of  primary  physio- 
logical conditions  of  excretion.  Some  or  all  of 
these  conditions  will  recur  independently  in  each 
of  the  phyla  of  the  animal  kingdom.  To  meet 
them,  the  necessary  cellular  elements,  till  then 
held  in  abeyance,  will  be  forthcoming. 

Bewildering  as  may  be  the  striking  identity 
which  Goodrich  has  established  between  the 
solenocytes  of  Amphioxus  and  of  Polychaetes,  it 
is  perhaps  not  more  remarkable  in  principle  than 
other  cases  of  identical  anatomical  differentiation, 
some  of  which  have  been  referred  to  in  the 
preceding  chapters. 

As  might  have  been  expected,  this  case  of 
nephridial  convergence  has  been  hailed  with 
delight  by  Dr  Gaskell,  who  looks  upon  it  as 
an  acceptable  confirmation  of  his  deductions.  I 
may  be  permitted  to  make  the  following  quota- 
tion from  his  book,  whilst  at  the  same  time 


SOLENOCYTES  157 

strongly  recommending  the  reader,  if  he  has 
not  done  so  already,  to  refer  to  the  original 
volume,  which  is  a  very  remarkable  document 
of  research.1  The  citation  will  serve  to  illustrate 
Dr  Gaskell's  point  of  view  in  this  matter,  and 
I  make  no  further  comment  upon  it,  relying 
upon  what  has  gone  before — except  to  point 
out  that  it  is  not  quite  correct  to  say  that  the 
Polychaeta  as  a  whole  are  the  highest  forms  of 
Annelida.  It  is  precisely  among  the  Polychaeta 
that  the  nervous  system  is  often  in  contact 
with  the  epidermis,  and  it  is  here  that  the 
cerebral  ganglion  retains  its  primitive  position 
in  the  prostomium. 

This  is  what  he  says : — "  It  is  to  me  most 
interesting  to  find  that  the  very  group  of 
Annelids,  the  Polychaeta,  which  possess  soleno- 
cytes  so  remarkably  resembling  those  of  the 
excretory  organs  of  Amphioxus,  are  the  highest 
and  most  developed  of  all  the  Annelida.  I 
have  argued  throughout  that  the  law  of  evolu- 
tion consists  in  the  origination  of  successive 
forms  from  the  dominant  group  then  alive, 
dominance  signifying  the  highest  type  of  brain- 
power achieved  up  to  that  time."  The  ways  of 
evolution  are  obscure  and  peculiar ;  logically 
one  would  think  that  they  ought  to  keep 
pace  with  the  increase  of  brain-power ;  naturally 

1  W.  H.  Gaskell,  "The  Origin  of  Vertebrates,"  London,  1908, 
see  p.  395. 


158     CONVERGENCE    IN   MINUTE   STRUCTURE 

we   find   that    the    increase   of    brain  -  power   is 
merely  an  incident  in  evolution. 

It  will  be  appropriate  at  this  point  to  consider 
another  example  of  what  I  hold  to  be  histo- 
genetic  convergence  as  between  the  lateral  sense- 
organs  of  the  Polychaete  family  of  the  Capitellidse, 
the  abdominal  sense-organs  of  Lamellibranchiate 
Molluscs,  and  the  lateral  line  sense-organs  of 
Vertebrates,  thus  involving  three  distinct  phyla. 
The  lateral  organs  of  Capitellidae  are  absolutely 
comparable  to  the  lateral  line  organs  of  Verte- 
brata,  but  only  by  way  of  convergence,  not  by 
way  of  homology. 

Let  us  first  look  at  the  abdominal  sense-organs 
of  the  bivalve  molluscs,  about  which  I  am  able  to 
speak  with  some  authority  inasmuch  as,  in  one 
instance,  namely,  that  of  the  Windowpane  Oyster, 
Placuna  placenta,  I  have  found  the  true  unpaired 
abdominal  sense  -  organ  which  had  been  over- 
looked by  previous  investigators.  The  story  of 
our  knowledge  of  these  sense-organs  is  interest- 
ing. The  year  1881  saw  a  considerable  advance 
in  the  morphology  of  the  Mollusca  in  consequence 
of  the  publication  of  Spengel's  paper  on  their  so- 
called  olfactory  organs,  subsequently  called  the 
osphradia  by  Lankester,  and  on  the  nervous 
system.  In  dealing  with  the  Lamellibranchiata 
he  was  at  first  at  a  loss  where  to  look  for  them, 
but,  thanks  to  a  lucky  chance  ("  einem  gliicklichen 
Zufalle  "),  on  opening  an  Area  noa  he  saw  at  once, 


ABDOMINAL   SENSE-ORGANS  159 

between  the  hinder  end  of  the  foot  and  the  vent, 
a  transverse,  undulating  line  of  greenish-brown 
pigment  interrupted  in  the  middle  line  by  a 
narrow  interval.  That  line  proved  to  mark  the 
position  of  the  pigmented  sense-organs  for  which 
he  was  searching.  In  the  figure  which  he  gave 
to  elucidate  the  topography  of  these  organs, 
another  pair  of  organs,  on  either  side  of  the  vent, 
was  indicated  with  great  distinctness,  although  no 
reference  was  made  to  them  in  the  text.  These 
latter  organs  were,  eight  years  later,  shown  by 
J.  Thiele1  to  represent  a  pair  of  abdominal 
sensory  papillae  having  a  highly  characteristic 
structure  and  innervated  by  a  fine  nerve  pro- 
ceeding backwards  on  each  side  from  the  visceral 
ganglion. 

When  examined  in  toto  in  a  fresh  preparation 
the  abdominal  sense-organs  are  chiefly  char- 
acterised by  the  possession  of  a  dense  coating 
of  long,  motionless,  stiff  cilia  or  sense-hairs.  In 
transverse  section  these  cilia  are  seen  to  be 
carried  by  a  very  high  epithelium  containing 
numerous  basal  nuclei  at  different  levels,  an 
intermediate  layer  of  nuclei  at  one  level,  and  a 
clear  peripheral  zone.  Thiele  described  the 
organs  in  a  number  of  bivalves,  including  a 
species  of  scallop,  Pectcn  varius,  where  he  found 
a  sensory  ridge  on  the  right  side  only,  there 
being  no  corresponding  ridge  on  the  left  side 

1  Zeitschr.  wiss.  Zool.,  xlviii.,  1889,  pp.  47-59. 


160  CONVERGENCE  IN  MINUTE  STRUCTURE 

in  this  species.  Similar  relations  have  been 
described  by  Dakin 1  in  another  species  of  scallop, 
Pectcn  maximus,  and  the  unpaired  condition  of 
the  organ  is  obviously  a  character  of  the  genus 
Pec  ten. 

In  the  Oriental  disc-shaped  bivalve,  P^tna 
placenta,  belonging  to  the  family  Anomiidae, 
only  one  sensory  abdominal  ridge  occurs.  It  is 
defined  by  the  presence  of  brown  pigment,  and 
is  placed  upon  the  edge  of  an  adrectal  ligament 
or  cutaneous  fold  which  unites  the  rectal  complex 
with  the  right  mantle  lobe  at  about  the  level  of 
the  posterior  insertion  of  the  suspensory  ligament 
of  the  right  ctenidium.  The  pallial  circulation 
of  Placuna  is  characterised  by  the  presence  of 
a  pair  of  ascending  pallial  vessels,  which  arise 
from  the  circumpallial  arteries  at  the  postero- 
ventral  border  of  each  mantle  lobe  and  pass 
obliquely  upwards  towards  the  posterior  end  of 
the  suprabranchial  region.  On  approaching  the 
latter  their  walls  are  usually  inflated,  after  the 
valves  have  been  separated,  to  form  an  elongated 
oblong  or  elliptical  body  which  is  non-contractile. 
I  do  not  know  the  precise  significance  of  this 
congestion  of  the  walls  of  the  ascending  pallial 
vessels  of  Placuna,  but  it  concerns  the  vascular 
system  and  has  nothing  to  do  with  the  adrectal 
sensory  organ. 

The  abdominal  sense-organ  of  Placuna  con- 

1  Memoir  on  Pecten.     Liverpool,  1909. 


SENSE-ORGAN   OF   PLACUNA  161 

forms  in  its  general  histological  structure  to  those 
previously  described  in  other  Lamellibranchs,  and 
I  have  had  the  opportunity  of  comparing  it  in 
section  with  the  paired,  whitish,  adrectal,  semi- 
lunar  organs  of  Area  rhombea,  a  species  which 
occurs  in  the  same  localities  as  Placuna  in  the 
back-waters  of  the  eastern  province  of  Ceylon, 
near  Trincomalee.  Transverse  sections  of  the 
sense-organ  in  Placuna,  stained  with  alum  car- 
mine, show  externally  the  dense  layer  of  perfectly 
preserved,  stiff,  sensory  cilia,  exceeding  in  length 
the  height  of  the  sensory  epithelium.  At  their 
bases  they  perforate  the  stout,  cuticular  mem- 
brane, which  presents  a  double  contour.  Below 
the  cuticle  there  follows  an  outer  prismatic  layer, 
then  a  middle  layer  of  evenly  placed  elliptical, 
nuclear  bodies  representing  the  layer  of  spindles  ; 
thirdly,  an  inner  fibrillar  layer,  and  finally  a  basal 
plexus  of  nuclear  bodies  adjoining  the  basement 
membrane  which  separates  the  epithelium  from 
the  gelatinous  conjunctive  tissue  upon  which  it 
rests. 

Thiele  emphasised  the  extraordinary  resem- 
blance between  the  epithelium  of  the  Molluscan 
abdominal  organs  and  the  Annelid  lateral  organs 
as  described  by  Eisig  in  the  Capitellidae ;  and 
satisfied  himself  by  actual  comparison  of  prepara- 
tions that  in  fact  the  similarity  is  very  close. 

Recently  another  pair  of  sense-organs,  histo- 
logically  resembling  the  abdominal  sense-organs 


162     CONVERGENCE   IN   MINUTE   STRUCTURES 

though  quite  different  topographically,  has  been 
described  by  M.  Stenta1  in  a  Protobranchiate 
bivalve  mollusc.  These  are  the  marginal  pallial 
sense-organs  of  Leda  commutata  (Nuculidae)  which 
occur  right  and  left  at  the  anterior  junction  of 
the  right  and  left  mantle  lobes,  lying  in  a  crypt 
on  each  side  between  the  inner  and  middle  folds 
of  the  pallial  margin.  If  any  one  ventured  to 
argue  that  the  posterior  adrectal  or  abdominal 
organs  of  Lamellibranchiata  might  be  related  by 
way  of  homology  to  the  segmental  abdominal 
organs  of  Capitellidae,  the  same  argument  could 
not  be  applied  to  the  anterior  pallial  organs  of 
Leda,  although  all  these  structures  apparently 
belong  to  one  physiological  category,  namely, 
rheostatic  organs.  We  may  confidently  conclude 
that  they  are  related  to  each  other  only  by  way 
of  sensory  convergence.  In  this  connection  we 
may  take  note  of  the  well-known  case  of  retinal 
convergence  between  the  pallial  eyes  of  the  scallop 
and  the  cerebral  eyes  of  vertebrates. 

Eisig2  treated  the  lateral  sense-organs  of  Capi 
tellidae  under  two  categories  —  thoracic  organs 
appearing  in  surface  view  as  open  pores ;  and 
abdominal  organs,  appearing  as  retractile  knobs  or 
elevations,  the  distal  extremity  of  which  is  beset 
with  stiff  sense  -  hairs  radiating  in  all  directions. 

1  Mario  Stenta,  "Uber  ein  neues  Mantelrandorgan  bei  Leda 
commutata?  ZooL  Anz.,  xxxv.,  1909,  pp.  154-157. 

2  Hugo  Eisig, "  Die  Capitelliden."    "  Fauna  und  Flora  des  Golfes 
von  Neapel."    Berlin,  1887. 


SENSE-ORGANS  OF  CAPITELLID^E  163 

These  sense  -  organs  were  closely  compared  by 
Eisig  with  the  lateral  line  sense-organs  of  aquatic 
vertebrates  (fishes  and  batrachians),  the  function 
of  which  is  partly  equilibrating  and  partly  the 
perception  of  currents  and  wave  -  movements. 
The  most  remarkable  histological  resemblance  is 
manifested  between  the  lateral  sense-organs  of 
the  Capitellidae  and  the  lateral  line  sense-organs 
of  Vertebrates.  In  both  cases  the  essential  organs 
consist  of  small,  solid,  roundish,  epidermal  buds, 
from  which  fine  stiff  sense  -  hairs  project  freely 
into  the  surrounding  medium  ;  and  the  resem- 
blance is  further  enhanced  by  their  segmental 
arrangement.  The  correspondence  could  hardly 
be  greater,  the  convergence  could  hardly  be 
closer,  the  homology  could  not  be  more  remote 
than  infinity. 

Within  the  limits  of  the  order  of  Polychseta, 
Eisig  has  established  the  true  homology  between 
the  lateral  sense-organs  of  Capitellidae  and  the 
dorsal  cirri  of  neural  parapodia  such  as  occur  in 
the  family  Glyceridae.  Through  this  family  it  is 
made  clear  that  the  lateral  organs  are  homologous 
with  the  dorsal  cirri  of  Polychseta  as  a  whole. 

Most  morphological  arguments  work  both  ways, 
and  it  is  not  surprising  that  after  having  estab- 
lished the  homology  between  the  endostyle  of 
Amphioxus  and  the  Ascidians  and  the  thyroid 
gland  of  Ammocoetes,  the  larval  form  of  the 


164     CONVERGENCE   IN   MINUTE   STRUCTURES 

lamprey,  Dohrn,  committed  to  the  Annelid  Theory 
of  vertebrate  descent,  should  have  concluded 
that  the  endostyle  was  derived  by  descent  with 
simplification  from  the  more  elaborate  organ  in 
Ammocoetes,  in  which  opinion  he  has  been  sup- 
ported by  Gaskell.  Regressive  convergence  by 
reduction  and  loss  of  parts  is  indeed  a  very 
common  phenomenon,  expressive  of  the  level- 
ling effects  of  degeneration,  e.g.,  the  loss  of 
limbs  in  snakes,  some  lizards,  some  batrachians, 
and  some  fishes ;  compare  also  the  effects  of 
sedentary  and  parasitic  habits.  In  less  obvious 
cases  this  principle  has  to  be  applied  with  the 
utmost  circumspection. 

For  Gaskell's  views  on  the  homology  which 
he  supposes  to  exist  between  the  thyroid  gland 
of  Ammocoetes  and  the  uterus  of  the  scorpion, 
the  reader  should  consult  the  work  quoted  above. 
For  my  present  purpose  the  following  passage 
will  suffice  (op.  cit.,  p.  205) : — "  The  resemblance 
between  the  structure  of  the  thyroid  of  Ammo- 
coetes and  the  uterus  of  the  scorpion  is  most 
striking,  except  in  two  respects,  viz.,  the  nature 
of  the  lining  of  the  non-glandular  part  of  the 
cavity — in  the  one  case  ciliated,  in  the  other 
chitinous — and  the  place  of  exit  of  the  cavity, 
the  thyroid  of  Ammocoetes  opening  into  the 
respiratory  chamber,  while  the  uterus  of  Scorpio 
opens  direct  to  the  exterior."  It  may  be  added 
by  way  of  explanation  that  the  supposed  homology 


MESOSOMA  165 

is  founded,  in  the  first  instance,  upon  the  mutual 
topographical  proximity  of  the  respiratory  appar- 
atus, and  of  the  terminal  portion  of  the  genital 
apparatus  in  the  scorpion  group  and  in  Limulus 
the  King  Crab,  and  the  relations  of  these  organs 
to  the  segments  of  the  mesosoma. 

It  is  not  easy  to  deal  with  this  comparison 
in  a  satisfactory  manner.  It  seems  to  be  some- 
what gratuitous  and  to  proceed  from  a  fallacious 
assumption  of  community  of  regional  differentia- 
tion. We  have  seen  in  several  instances  that 
identity  of  structure  and  function  may  go  for 
nothing  in  determining  homology,  and  that  the 
same  identity  has  no  relation  to  topography.  If 
one  chooses  to  compare  the  respiratory  region 
of  the  scorpion  or  of  Limulus  with  that  of 
Ammocoetes,  and  to  dub  it  mesosoma  in  both 
cases,  all  sorts  of  curious  sequences  will  be 
encountered.  Without  a  preconceived  bias,  such 
as  Dr  Gaskell  does  not  conceal,  namely,  in 
respect  of  his  ideas  upon  dominance  and  brain- 
power, one  would  not  consider  the  two  regions 
comparable  morphologically.  It  is  a  very  good 
thing  to  have  a  guiding  idea  in  morphology  and 
to  follow  it  out,  but  at  the  best  it  can  only  lead 
to  a  subjective  conclusion.  There  is  no  necessity 
to  confound  such  a  conclusion  with  the  truth, 
and  this  is  all  we  can  ask,  since  the  reconcilia- 
tion of  the  truth  with  one's  individual  views 

is  not  a  matter  which  can  be  settled  within  a 

L  2 


166      CONVERGENCE   IN   MINUTE   STRUCTURE 

generation.  At  the  same  time  it  would  seem 
proper  to  insist  here  that  in  the  face  of  an 
assertive  logical  system  which  can  only  be  com- 
bated at  the  expense  of  infinite  weariness  and 
vexation,  the  best  and  only  course  is  to  hold 
fast  to  what  one  adopts  intuitively  as  first 
principles ;  and,  with  regard  to  the  special  case 
before  us,  to  regard  the  gill-cleft  as  an  autonomous 
morphon  and  the  Limuloid  gill-book,  with  its 
derivative  the  lung-book  of  Arachnida,  as  another 
autonomous  morphon,  having  nothing  in  common 
except  their  function. 

The  histological  resemblance  between  the 
glandular  part  of  the  thyroid  of  Ammocoetes 
and  the  glandular  part  of  the  uterus  of  the 
scorpion  may  be  regarded  as  a  case  of  glandular 
convergence,  and  as  such  it  possesses  a  peculiar 
interest  of  its  own. 

Now  if  we  reject  the  Limuloid  Theory  of 
Vertebrate  descent,  why  should  we  accept  as 
normal  the  theory  of  the  Arachnoid  affinities 
of  Limulus  ?  The  answer  in  brief  is  that  the 
normal  morphology  of  Arachnoids  and  of  Verte- 
brates respects  phyletic  boundaries ;  and  that 
the  convergent  morphology  of  these  groups  tran- 
scends those  limits.  Both  are  good  so  long  as 
they  are  kept  apart. 

It  would,  of  course,  be  possible  to  multiply 
examples  of  histological  parallelism  to  an  almost 
unlimited  extent  by  comparing  distantly  related 


CYTOLOGY  167 

forms  and  making  extracts  from  published 
records.  All  animals  are  related  together  by 
the  continuity  of  the  germ-plasm  ;  the  remoter 
the  relationship,  the  closer  the  convergence  may 
be,  and  vice  versa.  An  instance  of  nuclear  con- 
vergence has  been  noted  recently  by  Minchin.1 
In  the  collar-cells  of  some  calcareous  sponges 
(Clathrinidce)  he  found  that  the  nucleus  occupies 
a  position  at  the  base  of  the  cell,  and  the 
flagellum  arises  independently  from  a  granule 
or  blepharoplast  situated  at  the  surface  of  the 
cell  in  the  centre  of  the  area  enclosed  by  the 
collar.  In  the  Leucosoleniidae  the  nucleus 
occupies  an  apical  position  and  the  flagellum 
appears  as  a  direct  continuation  of  the  pointed 
end  of  the  nucleus. 

Minchin  quotes  a  parallelism  to  these  alter- 
native positions  of  the  nucleus  in  the  case  of  two 
species  of  Mastigina  described  by  Goldschmidt 
(1907).  Such  a  character,  adds  Minchin,  in  the 
case  of  sponges,  can  have  but  little  importance 
in  the  struggle  for  existence,  and  yet  in  his 
opinion  it  indicates  the  deepest  phylogenetic 
divergence  in  the  pedigree  of  the  calcareous 
sponges. 

Cytological  convergence,  as  between  Metazoa 
and  Protozoa,  yields  many  points  of  instructive 

1  E.  A.  Minchin,  "The  Relation  of  the  Flagellum  to  the  Nucleus 
in  the  Collar-Cells  of  Calcareous  Sponges."  ZooL  Anz.,  xxxv.,  1909, 
p.  227. 


1 68      CONVERGENCE   IN   MINUTE   STRUCTURE 

comparison.  The  phenomena  of  intracellular 
digestion,  phagocytosis,  and  wandering  cells  come 
under  this  category.  An  elaborate  case  is  that 
of  the  pulsating,  multinucleated  chromatophores 
of  eight-armed  Cephalopoda.  In  Octopus,  under 
certain  conditions  of  excitement,  the  play  of 
colour  may  be  observed  taking  place  with 
marvellous  regularity  and  rapidity.  Each  chrom- 
atophore,  as  Chun1  has  demonstrated,  is  a  single 
disc-shaped  cell  with  contractile  processes  radi- 
ating from  the  periphery ;  and  these  contractile 
processes  originate  from  the  cell-body  in  the 
same  manner  as  the  pseudopodia  of  a  rhizopod 
Protozoan. 

The  preceding  examples  bear  witness  to  the 
truth  of  the  conclusion  that  convergence  depends 
equally  upon  the  unity  of  plan  of  composition 
of  the  body  of  animals,  as  indicated  in  the 
incidence  of  planes  of  symmetry,  and  upon  the 
continuity  of  the  system  of  functions  from 
Protozoa  to  Metazqa. 

Now,  if  we  stop  for  a  moment  to  enquire  what 
is  the  bearing  of  all  this,  it  may  be  said  that  I 
have  failed  in  my  purpose  unless  it  has  been 
made  abundantly  clear  that  the  influence  of 
convergence  in  evolution  has  been  widespread, 

1  Carl  Chun,  "  Uber  dieNatur  und  die  Entwicklung  der  Chrom- 
atophoren  bei  den  Cephalopoden."  Verh.  d.  Deutschen  Zool.  Ges., 
1902,  pp.  162-182. 


MORPHONS   AND   TROPISMS  169 

deep-seated,  and  intimate,  more  so  than  is 
generally  recognised.  What  may  appear  to  be 
a  brilliant  discovery  of  morphological  affinity 
may  in  reality  be  an  equally  brilliant  demonstra- 
tion of  the  no  less  important  and  interesting 
phenomenon  of  morphological  convergence ;  the 
closer  the  identity,  as  between  forms  belonging 
to  different  phyla,  the  greater  likelihood,  or,  as 
I  should  prefer  to  say,  the  greater  certainty  that 
it  is  due  to  convergence. 

What  is  known  as  homoplasy  in  morphology 
might  be  called  homotaxis  in  bionomics.  In 
contrast  with  the  phenomenon  of  change  of 
function  we  have  that  of  substitution  of  organs, 
as  Kleinenberg  expressed  it,  or  change  of 
morphon  in  Spengel's  phraseology.  Similarly, 
in  contrast  with  the  Tropism  Theory  we  must 
have  the  Morphon  Theory,  and  we  must  dis- 
tinguish between  primary  or  general  morphons 
which  are,  with  due  reserves,  the  intrinsic 
property  of  all  animals,  and  secondary  or 
phyletic  morphons  which  are  the  special  char- 
acteristics of  distinct  groups.  The  relation  of 
morphons  to  tropisms  lies  at  the  basis  of  all 
orthogenetic  morphology  and  all  convergent 
morphology.  Under  the  one  or  the  other  head- 
ing all  pertinent  facts  can  be  ranged,  and  none 
need  be  left  out  of  the  reckoning. 

It  will  be  observed  that  no  attempt  has  been 
made  in  the  foregoing  pages  to  formulate  any  laws 


170      CONVERGENCE   IN   MINUTE   STRUCTURE 

of  orthogenesis  and  convergence.  A  greater 
assemblage  of  facts  than  has  been  marshalled 
here  would  be  necessary,  nothing  short  of  a 
new  cyclopaedia  of  anatomy  and  physiology,  and 
probably  the  time  is  not  yet  ripe  for  that.  Up 
to  the  present  no  other  work  with  which  I 
am  acquainted  has  dealt  with  convergence  as  a 
general  and  positive  phenomenon  of  equal  import- 
ance with  orthogenesis  or  normal  morphology, 
although  isolated  cases  are  referred  to  in  most 
text-books  of  zoology  and  are  exhibited  in  most 
natural  history  museums. 

The  fact  that  no  laws  of  convergence  are  or 
even  can  be  laid  down  now  is  one  which  is 
fraught  with  the  greatest  hope  for  the  future 
of  morphology  ;  and  the  breaking  down  of  the 
former  landmarks  of  homology,  such  as  histo- 
logical  structure  and  metameric  repetition,  except 
within  narrow  limits,  offers  a  great  opportunity 
for  emancipation  from  the  trammels  of  specu- 
lation. If  we  were  to  tabulate  laws  they  would 
not  be  natural,  but  merely  dogmatic,  at  the  mercy 
of  the  first  unbeliever.  Hardly  one  universal 
criterion  of  strict  homology  can  be  mentioned 
which  would  pass  muster  in  a  critical  examina- 
tion. Then  away  with  laws  and  away  with 
criteria  until  they  cease  to  obscure  the  facts  as 
they  are. 

Hypotheses  are  one  of  the  chief  means  of 
progress  in  morphology.  Without  them  the 


CONCLUSION  171 

advances  which  have  been  made  during  the  past 
hundred  years  would  not  have  been  so  consider- 
able. If  the  hypothesis  is  constructed  after  the 
work  is  done  or  before  it  is  completed,  the  work 
accomplished  remains  after  the  need  for  the 
hypothesis,  which  is  either  the  best  clue  that 
one  can  give  for  the  time  being  or  the  best 
guide  that  one  can  follow,  has  passed  away. 
In  morphology  everything  is  important  except 
the  hypothesis,  although  practically  nothing  could 
be  done  without  it,  since  it  is  often  the  only 
means  available  for  digesting  an  accumulation 
of  facts.  It  is  something  intangibly  necessary, 
often  quite  wrong,  always  hopelessly  incom- 
plete, but  ever  ready  to  give  way  by  substitution 
to  another  invisible  vehicle.  The  progress  of 
morphology  depends  upon  the  substitution  of 
ideas  rather  than  upon  the  promulgation  of  laws. 
The  tree  of  life  is  polyphyletic,  and  the  branches 
do  not  anastomose  after  their  zigzag  course  has 
been  set. 


INDEX 


ABDOMINAL  sense-organs,  159 

Accessory  branchial  organs,  141 

Acorn  barnacles,  39 

Air-bladder,  144,  151 

Air-breathers,  19,  141 

Alciopidae,  44 

Alcock,  A.,  106,  117 

Alytes,  97,  134 

Amia,  135 

Ammocoetes,  163 

Amnion,  16 

Amphibia,  19,  142 

Amphinome,  40 

Amphioxus,  21,  45,  46,  72,  85,  139, 

151,  153,  163 
Ampullaria.)  18 
Anabas,  141 

Analogy  (see  Parallelism) 
Annelid  Theory,  164 
Annelida,   43,   77,   118,   129,   139, 

153,  157,  161 

Anterior  neuropore,  83,  85 
Ants,  63,  114,  119 
Appcndicularia,  38 
Appendiculata,  70,  78,  82,  125 
Aquatic  animals,  17 
Arachnoidea,  148,  166 
Arboreal  mammals,  28,  57 
Area,  158,  161 
Archenteron,  77 
Arius,  131,  137,  146 
Arixenia,  115 
Arthropoda,  148 
Ascidians  (see  Tunicata) 
Asphyxiation,  18 
Aspredo,  133 
Assheton,  R.,  144 
Atheston,  L.,  119 
Auditory  organs,  82 
Autotomy,  63 

BALANOGLOSSUS      (see      Entero- 

pneusta) 
Balanus,  39 


Balfour,  F.  M.,  72 
Barnacles  (see  Cirripedes) 
Batesian  mimicry,  55 
Bateson,  W.,  72,  97,  122 
Bather,  F.  A.,  36 
Batrachia,  19,  92,  96 
Bats,  23,  89,  105 
Beddard,  F.  E.,  46 
Beebe,  C.  W.,  60 
Belonc,  155 
Belostomatidge,  134 
Biocoenosis,  114 
Birds,  89,  123,  151 
Birgus,  1 8 
Blenny,  140 
Boake,  B.,  131,  141 
Boeke,  J.,  136 
Bois-Reymond,  E.  du,  51 
Boulenger,  C.  L.,  132 

,  G.  A.,  95,  102 

Boveri,  Th.,  153 
Brain-power,  103,  157,  165 
Branchiata,  148 
Branchiotrema,  150 
Brood-nursing  (see  Incubation) 
Bruce,  W.  S.,  100 
Buckland,  F.  T.,  22,  30,  52,  92 
Budge tt,  J.  S.,  136 
Bufonidse,  102 
Bulbuls,  22 
Burne,  R.  H.,  145 
Burrowing  mammals,  29,  56,  80 
Butterflies,  27,  59 

CALLICHROMA,  55 

Calotes,  22,  50 

Camper,  P.,  152 

Capitellidse,  158,  162 

Cartilaginous  fishes,  94 

Cat's  eye,  92 

Cave,  W.  A.,  59 

Cell-theory,  4 

Cephalisation,  70,  113 

Cephalopoda,  33,  70,  95,  137,  168 


173 


INDEX 


Cerebration,  114 

Ceylon,   45,    55,    59,   6l,   64,    88, 

109,  131,  134,  HO,  145,  161 
Chameleon,  52 
Chank  shell,  98 
Chatoessus,  no 
Chironomus,  118 
Chlorophyll,  26 
Chordata,  72,  150 
Chromatism,  27 
Chromatophores,  1 68 
Chun,  C,  168 
Cimex,  61 
Cirripedes,  39,  116 
Clarias,  141 

Clupeidae,  95,  109,  112,  145 
Cobra,  30 
Ccelenterata,  70,  78 
Coelom,  77,  122 
Coelomata,  77 
Collar-cells,  167 
Collin,  A.,  129 
Cooke,  A.  H.,98 
Crabs  (see  Crustacea) 
Crocidura^  79 
Crows,  25 
Crustacea,  52,  95 
Cryptotaxis,  33 
Ctenidium,  147 
Cutaneous  incubation,  133 
Cuttlefishes  (see  Cephalopoda) 
Cuvier,  Baron,  4,  13,  80 
CylindrophiS)  49 

DACTYLOPTERUS,  88 

Dahl,  F.,  7,  114 

Dakin,  W.  J.,  44,  160 

Darwin,    C.,    I,    5,   67,    112,    116, 

132 

Day,  F.,  131 
Dean,  B.,  135 
Death-feint,  66 
Dendy,  A.,  126 

Dewitz, ,  38 

Diopatra,  43 
Dipnoi,  19,  142 
Direct  development,  122 
Distant,  W.  L.,  134 
Dohrn,  A.,  83,  164 
Dominance,  157,  165 
Dorsalcirri,  163 
Draco,  50 
Dryophis,  93 
Duerden,  J.  E.,  20 
Dussumieria,  95,  109 

ECHINODERMATA,  36,  71,  122 


Ectoparasites,  115 

Eisig,  H.,  43,  51,  162 

Electric  organs,  51,  91,  112 

Elephant,  27 

Endoskeleton,  14 

Endostyle,  163 

Enteropneusta,  32,  42,  72,  122,  123, 

128,  140,  151 
Entoprocta,  155 
Espada,  J.  de  la,  132 
Excretory  organs,  153-155 
Exoccetus,  88 
Exuviation,  52 
Eyelids,  95,  112 

FLAME-CELLS,  155 

Flatfishes,  97 

Fleas,  7,  61,  115 

Flower,  W.,  and  Lydekker,  R.,  79, 

91 
Flying  fishes,  88 

foxes,  24 

leaps,  50 

mammals,  57,  89 

Food-yolk  (see  Lecithality) 
Fox's  eye,  92 
Frog,  42,  92 
Froggart,  W.  W.,  120 
FusuSy  97 

GADOW,  H.,  102 

Galeopithecus,  57,  91 

Gaskell,  W.  H.,  73,  75,  81,  82,  85, 

103,  156,  164 

Gastral  convergence,  107-112 
Gastropoda,  41,  149 
Geckoes,  22,  92 
Gill-book,  1 66 

Gill-clefts,  139,  144,  149-151,  1 66 
Gills,  cutaneous,  139 
Gizzard,  107,  no 
Glandular  convergence,  106,  166 
Globe-fishes,  144 
Glyceridse,  163 
Goat's  eye,  93 
Goby,  140 

Goldschmidt,  R,,  167 
Goodrich,  E.  S.,  154 
Green,  E.  E.,  59 
Grey  mullet,  109 
Groom,  T.  T.,  41 
Gruvel,  A.,  41 
GUnther,  A.,  64,  133,  134 
Gut,  75,  155 
Gymnarchus,  137,  144 

H^MADIPSA,  45 


INDEX 


Hsematotropism,  48 

Haemoglobin,  118 

Harvey,  W.,  151 

Hawk-moths,  23 

Hedgehog,  30,  105 

Heliotropism,  20,  41 

Hemichordata,  72 

HcmimeruS)  115 

Herpestes,  94 

Herring,  146 

Heteropoda,  41 

Heterotypic  convergence,  90 

Hickson,  S.  J.,  140 

Hippocampus,  52,  68 

Hipponoe,  40 

Hirudo,  45 

Histology,  153 

Hodgson,  T.  V.,  99 

Homing,  25 

Homology,  53,  75,  *5i»  *S2>  I55y 

163 

Homoplasy,  u,  12,  53,  57,  90,  169 
Homotaxis,  169 
Howes,  G.  B.,  91,  132 
Hubrecht,  A.  A.  W.,  16,  94,  103- 

105,  125,  144 
Hunter,  J.,  106,  in,  151 
Huxley,  T.  H.,  13,  141 
Hydroid  polyps,  37,  117 
Hydromedusae,  37 
Hydrostatic  organs,  144 
Hydrus  platurusy  116 

ICHTHYOPHIS,  31,  143 
Incubation,  131-134,  142 
Infundibulum,  82,  83,  85 
Ink-sac,  33 
Insectivora,  79 

Insects,  58,  115,  118,  120,  139,  148 
Invertebrates,  13 
Izuka,  A.,  130 

JAPANESE  palolo,  130 
Jennings,  H.  S.,  20 
Jordan,  D.  S.,  51 

,  K.,  115 

Jumping  fishes,  140 
Jungle  life,  28 

KALLIMA,  s8 

Keith,  A.,  87 

Kerr,  J.  G.,  136,  143 

Kinetoblast,  128 

King  Crab  (see  Limulus) 

Kleinenberg,  N.,  169 

Korschelt  und  Heider,  40 

Kramer, ,  129 


LABYRINTHINE  organs,  141 
Lacaze-Duthiers,  H.  de,  84 
Lamarck,  13 
Lamellibranchiata,  158 
Land  crabs,  17 

leeches,  18,  31,  45 

mollusca,  17,  149 

nemertines,  17 

planarians,  17,  28 

Lang,  A.,  37 

Lankester,  Sir  E.  R.,  11,53,  !47» 

149,  158 

Larval  forms,  42,  122,  128 
Lateral  sense-organs,  163 
Leaf-fish,  66 
Leaf-mimicry,  58-66 
Leandcr,  40 
Lecithality,  124 
Lcda,  162 
Leigh,  H.  S.,  62 
Lcpas,  39,  117 
Lepidastkenia,  43 
Lepidosircn,  143 
Lepidosteus,  96 
Limnaeidae,  142 
Limnodrilus  (see  Tubificidae) 
Lo  Bianco,  S.,  139 
Loeb,J.,38 
Longitudinal  stripes,  68 
Lophobranchii,  68,  133 
Lungs,  139,  145 
Lycodon,  49 
Lydekker,     R.    (see    Flower    and 

Lydekker) 

MACBRIDE,  E.  W.,  72 

Mammals,  12,  16,  26,  29,  57,  68, 

80,  89,  91-93,  103-106,  125 
Mammary  glands,  105 
Afants,  104 
Marsupials,  56,  79,  80 
Mayer,  A.  G.,  130 
Medicinal  leech  (see  Hirudo) 
Meerkat,  94 
Megalops,  108,  145 
Mesosoma,  165 
Mesozoic  reptiles,  91 
Metamerism,  77,  78 
Metazoa,  15 
Metschnikoff,  E.,  123 
Miall,  L.  C,  118 
Milk,  107 
Mimicry,  52 
Minchin,  E.  A.,  167 
Minous  inermis,  117 
Mole-like  forms,  56 
Molgula,  84 


i76 


INDEX 


Mollusca,  17,  41,  77,  97,  139,  147, 

149,  158 

Monkeys,  28,  50 
Monotrema,  52,  68,  125 
Moore,  J.  P. ,  40 
Morphon,  144,  169 
Morris,  C,  144 
Moseley,  H.  N.,  125,  140 
Mugilidse,  112 
Muller,  J.,  123 
Mullet,  108 
Mungoose,  94 
MyobatraehuS)  102 
Myoepithelial  cells,  155,  156 
Myriopoda,  148 
Myrmecoidism,  121 
Myrtnedonta,  121 

NAUPLIUS,  39-41 

Nautilus,  33,  137 

Nematocysts,  155 

Nematodes,  70,  155 

Nemertines,  42,  77 

Nephridia,  154 

NcphthyS)  44 

Nervous  system,  75»  &2>  86,  I][3 

Newton,  A.,  23 

Newts,  52 

Nictitating  membrane,  94 

Notochord,  14,  72,  73 

Nucleus,  167 

Nuculidse,  162 

Nutrition,  73,  103,  126 

OESOPHAGUS,  83,  85,  156 
Olfactory  pit,  85 
Onychophora,  125 
Opercular  membrane,  141,  147 
Ophelia,  139 
Ophiocephalus,  135,  141 
Ophiodromus,  43 
Opisthobranchiata,  147,  149 
Opisthocoelous  vertebrae,  97 
Opposable  extremities,  95 
Osborn,  H.  F.,  12,  68 
Osphradium,  147,  158 
Osteophora,  94 
Oviparity,  124,  125 
Owen,  Sir  R.,  4,  93,  96,  106,  108, 
I5i 

PALLIAL  sense-organs,  162 

Palolo,  43,  129 

Pantopoda,  99 

Parachute  flight,  49,  90,  91 

Parallelism,  57,  70,  79,  128,  149 

Parasitism,  31,  74,  76 


Patella,  147 
Pecten,  44,  159,  160 
Pectoral  fins,  88 
Pelmatozoa,  37,  71 
Pentanymphon,  100 
Periophthalmus,  140 
Peripatus,  17,  125 
Phanerozoa,  23 
Phasmidse,  63 

Phosphorescence,  21,  34,  91,  1 12 
Phototaxis,  20 
Phyllidia,  147 
Phy Ilium,  61 
Phyllodoce,  154 
Phyllopteryx,  64 
Physiognomical  convergence,  57 
Pilidium,  42 
Pipa,  96,  133 

Pipe-fishes  (see  Lophobranchii) 
Placentation,  103-105,  126 
Placuna,  160 
Planozoic,  39 
Platax,  64,  66 
Platyhelminthes,  70,  77 
Platypus  (see  Monotrema) 
Pleotropism,  41,  43,  45,  122 
Plotosus,  147 
Pocock,  R.  I.,  63,  149 
Polychaeta  (see  Annelida) 
Polypedates,  134 
Polypterus,  145 
Poulton,  E.  B.,  54 
Prehensile  tails,  52,  68 
Protective  coloration,  26 
Protochordata,  72,  83 
Protozoa,  15,  167 
Psamtnolyce,  43 
Pteromys,  57,  91 
Pteroplatcea,  106 
Pteropoda,  41 
Pulmonate  Arachnida,  149 

Mollusca,  149 

Punnett,  R.  C,  150 
Pupil  of  the  eye,  93,  102 
Purcell,  W.  F.,  148,  149 
Pygmies,  87 
Python,  142 

RACIAL  convergence,  87 
Raptorial  appendages,  95 
Recapitulation,  42 
Regeneration,  46,  52,  63 
Regression,  125,  164 
Reptiles,  21,  $o,  124 
Respiration,  139 
Respiratory  pigments,  26 
Retina,  162 


INDEX 


177 


Reversed  shells,  97 

vertebrae,  97 

Rhacophorus ;  92,  95,  134 
Rheostatic  organs,  162 
Rhinoderma,)  132 
Rhinophrynus ,  102 
Rhodosoma,  84 
Rodentia,  56,  79 
Rotifers,  155 

SACCOBRANCHUS,  141,  145 

Salarias,  140 

Sarasin,  P.  and  F.,  142 

Sarcodaces,  136 

Sauropsida,  72,  137 

Schimkewitsch,  W.,  99 

Scombresocidae,  88,  155 

Scorpion,  29,  164 

Scylltza,  40 

Seasonal  flights,  27 

Sedgwick,  A.,  42,  87,  126 

Semper,  K.,  18,  37,  52,  88 

Sensory  convergence,  158-163 

Sessile  animals,  36 

Seton,  E.  T.,  92 

Shelford,  R. ,  49 

Siluridae,  131,  141 

Sinistral  shells,  98 

Situs  tnversus,  97 

Snakes,  49,  67,  93,  116,  164 

Social  insects,  119 

Solenocytes,  154-157 

Somites,  77 

Spengel,  J.  W.,  144,  158,  169 

Sphcerodema,  134 

Spiders,  148 

Sponges,  36,  167 

Spurious  convergence,  81 

Statozoa,  37 

Stenta,  M.,  162 

Stereotropism,  38,  122,  143 

Sting-rays,  1 06 

Strobilation,  77,  78 

Stylactis,  117 

Substitution,  14,  143,  169 

Surinam  toad  (see  Pipa) 

Swarming  habits,  43,  129 

Synaposematic,  55 

TADPOLE,  42 


Tagetis,  60 
Teats,  125 
Teeth,  12 
Teleostei,  88,  112 
Tennent,  Sir  E.,  48 
Termites,  63,  120 
Testacella,  31 
Thiele,  J.,  159,  161 
Thompson,  D'Arcy  W.,  99 
Thyroid  gland,  163,  164 
Tilapia  nilotica,  132 
Toads,  22,  92,  101 
Topography,  165 
Tornaria,  42,  123 
Tracheae,  148 
Tracheata,  148 
Triploblastica,  77 
Trophoblast,  16,  103,  128 
Tropism,  20,  22,  169 
Tubificidae,  118 
Tunicata,  72,  84,  155,  163 
Types,  9 

UEXKULL,  J.  von,  20 
Ungulates,  104 

VARANUS,  50 

Variation,  58,  60,  62 

Veliger,  41 

Vertebrae,  96 

Vertebrata,  13,  81,  85,  158 

Verworn,  M.,  21 

Viability,  146 

Visceral  clefts  (see  Gill-clefts) 

Viviparity,  124 

Viviparous  rays,  106 

WALLACE,  A.  R.,  53,  58 

Warning  coloration,  54 

Wasmann,  E.,  120,  121 

Water-breathers,  19 

Weiss,  F.  E.,  153 

Whitman,  C.  O.,  46 

Whitmee,  S.  J.,  129 

Willey,  A.,  16,  64,  124,   125,  135, 

ISO 

Windowpane-oyster,  158 

Wings,  89 


ZOOPHYTES,  35 


M 


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